bug#19040: 25.0.50; Building from Git leaves emacs-repo-version at nil

[Puneeth Chaganti]

Looks like we call `git log -1 --pretty=format:%N` to get the version, 
which looks for git notes, if any.

Instead calling `git log -1 --pretty=format:%h` would give the short 
hash.  We may also want to set a dirty flag, though.