View the log of the staged files while editing a git commit message

Question

When I'm editing a Git commit message, I sometimes like to browse recent commits to the same file(s). That is, I'd like to view the output of git log $list_of_files_with_staged_changes.

Git commit mode (from Magit) provides some nice features, including a history of my recent commit messages (M-p/M-n = git-commit-prev-message/git-commit-next-message). But these are only commits that I've done in the same session, not necessarily in the same repository. Here I only want to see commit messages that are relevant to the files I'm committing, and I want them no matter how old they are. I also want the commit IDs.

I also have Egg (Emacs's Got Git) installed, and it provides a quick way to access the log for all the files in the repository (C-x v l: egg-log). But that's too much noise.

While editing a Git commit message, how can I view the log of the files I'm committing? I can use both Egg and Magit if convenient, as well as the built-in vc-git.

Answer 1

The following function does the job in the nominal case, when committing files in the index.

(defun magit-log-staged-files (&optional args use-vc-log)
  "Show the change log of the files in the Git index.
If no files are staged in the index, log all changed files.

If USE-VC-LOG is non-nil (interactively: with a prefix argument),
call `vc-git-print-log' to show the log. Otherwise call `magit-log'."
  (interactive (list magit-log-arguments current-prefix-arg))
  (let ((files (or (magit-staged-files)
                   (magit-changed-files "HEAD"))))
    (if use-vc-log
        (vc-print-log-internal 'Git files "HEAD")
      (magit-log '("HEAD") args files))))
(define-key git-commit-mode-map "\C-c\C-l" 'magit-log-staged-files)

These aren't the right files when committing a set of files explicitly (git commit FILENAME…), which I think Magit doesn't do.