Force output format for `git show` to get info about HEAD commit

Information about the current HEAD may change when using default or any named format. To correctly parse it it's necessary to give an explicit format.

As an additional bonus, with an explicit format one does not need to do any pattern matching, because different information is in separate lines.
Merge pull request #2 from rewritten/explicit_format_for_git_log

Force output format for `git show` to get info about HEAD commit