Reduce number of files send for propose_translation

If a single file has changed, all other .pot files are send as well
since they have a new timestamp.

Check for files that only have a new timestamp and revert their change
so that they do not get committed at all.

Also, handle Project-Id-Version the same way as the timestamp.

Change-Id: I49488d01a7bb9242508ec7d378240449124608de
Merge "Reduce number of files send for propose_translation"