git-save: improve commit authorship data

We used to always just commit as "subsurface@hohndel.org" because
libgit-19 doesn't have the interfaces to do user name lookup.  This does
better if you have libgit-20, using "git_signature_default()" to get the
actual user that does the saving.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>