[BUG] git stash -k show the help message for diff-index

Everything is in the title. No time to bisect/fix this now, but:

$ git status
# On branch master
nothing to commit (working directory clean)
$ git stash -k
usage: git diff-index [-m] [--cached] [<common diff options>] <tree-ish> [<path>...]
common diff options:
  -z            output diff-raw with lines terminated with NUL.
  -p            output patch format.
  -u            synonym for -p.
  --patch-with-raw
                output both a patch and the diff-raw format.
  --stat        show diffstat instead of patch.
  --numstat     show numeric diffstat instead of patch.
  --patch-with-stat
                output a patch and prepend its diffstat.
  --name-only   show only names of changed files.
  --name-status show names and status of changed files.
  --full-index  show full object name on index lines.
  --abbrev=<n>  abbreviate object names in diff-tree header and diff-raw.
  -R            swap input file pairs.
  -B            detect complete rewrites.
  -M            detect renames.
  -C            detect copies.
  --find-copies-harder
                try unchanged files as candidate for copy detection.
  -l<n>         limit rename attempts up to <n> paths.
  -O<file>      reorder diffs according to the <file>.
  -S<string>    find filepair whose only one side contains the string.
  --pickaxe-all
                show all files diff when -S is used and hit is found.
  -a  --text    treat all files as text.

usage: git diff-index [-m] [--cached] [<common diff options>] <tree-ish> [<path>...]
common diff options:
  -z            output diff-raw with lines terminated with NUL.
  -p            output patch format.
  -u            synonym for -p.
  --patch-with-raw
                output both a patch and the diff-raw format.
  --stat        show diffstat instead of patch.
  --numstat     show numeric diffstat instead of patch.
  --patch-with-stat
                output a patch and prepend its diffstat.
  --name-only   show only names of changed files.
  --name-status show names and status of changed files.
  --full-index  show full object name on index lines.
  --abbrev=<n>  abbreviate object names in diff-tree header and diff-raw.
  -R            swap input file pairs.
  -B            detect complete rewrites.
  -M            detect renames.
  -C            detect copies.
  --find-copies-harder
                try unchanged files as candidate for copy detection.
  -l<n>         limit rename attempts up to <n> paths.
  -O<file>      reorder diffs according to the <file>.
  -S<string>    find filepair whose only one side contains the string.
  --pickaxe-all
                show all files diff when -S is used and hit is found.
  -a  --text    treat all files as text.

Saved working directory and index state WIP on master: 977c790 foo HEAD is now at 977c790 foo