git: Rephrase the description of Commit.short_hash.
git: Add a new attribute to Commit: msg (commit message).
git: Rename *_output variables to output.

The shorter names are more readable.
test_git: Remove duplicate code from CommitCreateAndAccessTests.
git: Refactor the obtaining of commits from git-show output.
git: Rename commit "message" to commit "subject".

We store only the first line of the commit message, so the name "subject" is more appropriate.
git: Use a custom git-show format to simplify the parsing of its output.
git: Merge _get_commit_from_show_output into _get_commit_from_git_show_with_args().
git: Rephrase a comment in Repo._get_commit_from_git_show_with_args().