maintner: intern git commit messages

Many revisions of git commits in Gerrit have the same commit message.
Share that memory.

Saves 11 MB (387 MB -> 376 MB).

Change-Id: I39f05cf309404f0b1d55c296a2fe551eb501ae9c
Reviewed-on: https://go-review.googlesource.com/38718
Reviewed-by: Kevin Burke <kev@inburke.com>
2 files changed