Merge branch 'kh/commit'

Merge branch 'kh/commit'

* kh/commit: (33 commits)
  git-commit --allow-empty
  git-commit: Allow to amend a merge commit that does not change the tree
  quote_path: fix collapsing of relative paths
  Make git status usage say git status instead of git commit
  Fix --signoff in builtin-commit differently.
  git-commit: clean up die messages
  Do not generate full commit log message if it is not going to be used
  Remove git-status from list of scripts as it is builtin
  Fix off-by-one error when truncating the diff out of the commit message.
  builtin-commit.c: export GIT_INDEX_FILE for launch_editor as well.
  Add a few more tests for git-commit
  builtin-commit: Include the diff in the commit message when verbose.
  builtin-commit: fix partial-commit support
  Fix add_files_to_cache() to take pathspec, not user specified list of files
  Export three helper functions from ls-files
  builtin-commit: run commit-msg hook with correct message file
  builtin-commit: do not color status output shown in the message template
  file_exists(): dangling symlinks do exist
  Replace "runstatus" with "status" in the tests
  t7501-commit: Add test for git commit <file> with dirty index.
  ...