Diagnostics function, colorized (I tried to add guards so it is portable with terminals that do not support color):

  git_diag() {
    local since="${1:-1 year ago}"
    local root repo branch

    # --- patterns ---
    local pattern="${GIT_DIAG_PATTERN:-fix|bug|broken|hotfix|incident|issue|patch}"
    local firefight_pattern="revert|hotfix|emergency|rollback"

    # --- colors ---
    local GREP_COLOR_MODE='never'
    if [[ -z "${NO_COLOR:-}" ]] && [[ -t 1 ]] && [[ "${TERM:-}" != "dumb" ]] && [[ "$(tput colors 2>/dev/null || echo 0)" -ge 8 ]]; then
      local BLACK=$(tput setaf 0)
      local RED=$(tput setaf 1)
      local GREEN=$(tput setaf 2)
      local YELLOW=$(tput setaf 3)
      local BLUE=$(tput setaf 4)
      local MAGENTA=$(tput setaf 5)
      local CYAN=$(tput setaf 6)
      local WHITE=$(tput setaf 7)
      local BOLD=$(tput bold)
      local DIM=$(tput dim 2>/dev/null || true)
      local RESET=$(tput sgr0)
      GREP_COLOR_MODE='always'
    else
      local BLACK='' RED='' GREEN='' YELLOW='' BLUE='' MAGENTA='' CYAN='' WHITE=''
      local BOLD='' DIM='' RESET=''
    fi

    local TITLE="$CYAN"
    local COLOR_COUNT="$CYAN"
    local COLOR_FILE="$YELLOW"

    if ! root="$(git rev-parse --show-toplevel 2>/dev/null)"; then
      printf 'git_diag: not inside a Git repository\n' >&2
      return 1
    fi

    repo="${root##*/}"
    branch="$(git branch --show-current 2>/dev/null)"
    branch="${branch:-DETACHED}"

    _git_diag_fmt_count() {
      local count_color="$1"
      local text_color="$2"

      awk -v count_color="$count_color" -v text_color="$text_color" -v reset="$RESET" '{
        c=$1
        $1=""
        sub(/^ +/, "")
        printf "  %s%10d%s  %s%s%s\n", count_color, c, reset, text_color, $0, reset
      }'
    }

    printf '%s%sGit repo diagnostics%s\n' "$BOLD" "$TITLE" "$RESET"
    printf '%s%-11s%s %s\n' "$BOLD" "Repo:"      "$RESET" "$repo"
    printf '%s%-11s%s %s\n' "$BOLD" "Branch:"    "$RESET" "$branch"
    printf '%s%-11s%s %s\n' "$BOLD" "Timeframe:" "$RESET" "$since → now"
    printf '\n\n'

    printf '%s%s1) Most changed files%s\n' "$BOLD" "$TITLE" "$RESET"
    git log --since="$since" --format='' --name-only \
      | awk 'NF' \
      | sort \
      | uniq -c \
      | sort -nr \
      | head -n 10 \
      | _git_diag_fmt_count "$COLOR_COUNT" "$COLOR_FILE"

    printf '\n%s%s2) Top contributors%s\n' "$BOLD" "$TITLE" "$RESET"
    git shortlog -sn --no-merges --since="$since" \
      | head -n 10 \
      | awk -v count_color="$COLOR_COUNT" -v reset="$RESET" '{
          printf "  %s%10d%s  %s\n", count_color, $1, reset, substr($0, index($0,$2))
        }'

    printf '\n%s%s3) Bug/fix hotspots%s %s(pattern: %s)%s\n' "$BOLD" "$TITLE" "$RESET" "$DIM" "$pattern" "$RESET"
    git log --since="$since" --format='' --name-only -i -E --grep="$pattern" \
      | awk 'NF' \
      | sort \
      | uniq -c \
      | sort -nr \
      | head -n 10 \
      | _git_diag_fmt_count "$COLOR_COUNT" "$COLOR_FILE"

    printf '\n%s%s4) Commit count by month%s\n' "$BOLD" "$TITLE" "$RESET"
    git log --since="$since" --format='%ad' --date=format:'%Y-%m' \
      | sort \
      | uniq -c \
      | sort -k2r \
      | awk -v count_color="$COLOR_COUNT" -v mag="$MAGENTA" -v reset="$RESET" '
        {
          data[NR,1] = $2
          data[NR,2] = $1
          if (length($1) > max) max = length($1)
        }
        END {
          for (i = 1; i <= NR; i++) {
            printf "  %s%10s%s  %s%*d commits%s\n",
              mag, data[i,1], reset,
              count_color, max, data[i,2], reset
          }
        }
      '

    printf '\n%s%s5) Firefighting commits%s %s(pattern: %s)%s\n' "$BOLD" "$TITLE" "$RESET" "$DIM" "$firefight_pattern" "$RESET"
    git log --since="$since" -i -E \
      --grep="$firefight_pattern" \
      --date=short \
      --pretty=format:'%ad %h %s' \
      | head -n 10 \
      | awk -v mag="$MAGENTA" -v dim="$DIM" -v reset="$RESET" '{
          date=$1
          hash=$2
          $1=$2=""
          sub(/^  */, "")
          printf "  %s%-10s%s  %s%-12s%s  %s\n",
            mag, date, reset,
            dim, hash, reset,
            $0
        }' \
      | GREP_COLORS='ms=01;31' grep --color="$GREP_COLOR_MODE" -i -E "$firefight_pattern"
  }

  git_diag() {
    local since="${1:-1 year ago}"
    local pattern="${GIT_DIAG_PATTERN:-fix|bug|broken|hotfix|incident|issue|patch}"
    local root repo branch

    if ! root="$(git rev-parse --show-toplevel 2>/dev/null)"; then
      printf 'git_diag: not inside a Git repository\n' >&2
      return 1
    fi

    repo="${root##*/}"
    branch="$(git branch --show-current 2>/dev/null)"
    branch="${branch:-DETACHED}"

    _git_diag_fmt_count() {
      awk '{
        c=$1
        $1=""
        sub(/^ +/, "")
        printf "  %10d  %s\n", c, $0
      }'
    }

    printf '============================================================\n'
    printf 'Git repo diagnostics\n'
    printf '%-11s%as\n' 'Repo:'      "$repo"
    printf '%-11s%s\n' 'Branch:'    "$branch"
    printf '%-11s%s\n' 'Timeframe:' "$since"
    printf '============================================================\n\n'

    printf '1) Most changed files (top 10)\n'
    git log --since="$since" --format='' --name-only \
      | awk 'NF' \
      | sort \
      | uniq -c \
      | sort -nr \
      | head -n 10 \
      | _git_diag_fmt_count

    printf '\n2) Top 10 contributors (no merges, since %s)\n' "$since"
    git shortlog -sn --no-merges --since="$since" \
      | head -n 10 \
      | _git_diag_fmt_count

    printf '\n3) Bug/fix hotspots (top 10, matching: %s)\n' "$pattern"
    git log --since="$since" --format='' --name-only -i -E --grep="$pattern" \
      | awk 'NF' \
      | sort \
      | uniq -c \
      | sort -nr \
      | head -n 10 \
      | _git_diag_fmt_count

    printf '\n4) Commit count by month (since %s)\n' "$since"
    git log --since="$since" --format='%ad' --date=format:'%Y-%m' \
      | sort \
      | uniq -c \
      | sort -k2r \
      | awk '
        {
          data[NR,1] = $2
          data[NR,2] = $1
          if (length($1) > max) max = length($1)
        }
        END {
          for (i = 1; i <= NR; i++) {
            printf "  %10s  %*d commits\n", data[i,1], max, data[i,2]
          }
        }
      '

    printf '\n5) 10 most recent firefighting commits (revert|hotfix|emergency|rollback)\n'
    git log --since="$since" -i -E \
      --grep='revert|hotfix|emergency|rollback' \
      --date=short \
      --pretty=format:'%ad %h %s' \
      | head -n 10 \
      | awk '{
          date=$1
          hash=$2
          $1=$2=""
          sub(/^  */, "")
          printf "  %-10s  %-12s  %s\n", date, hash, $0
        }' \
      | GREP_COLORS='ms=01;31' grep --color=always -i -E 'revert|hotfix|emergency|rollback'
  }