propose_translation_update: Handle .tx/config in git

Some packages have .tx/config in git. The tx commands update
.tx/config if new localization files are added to the repository.
If .tx/config changes, "git review" will fail with a
message stating that there are uncommitted files in the tree.

Take care that .tx/config is not changed and thus "git review" does not
fail.

Changes to .tx/config need to be done as part as separate changes.

Change-Id: I235877fb568c0365cc822c265d401f0cf4dfdbf5
Merge "propose_translation_update: Handle .tx/config in git"