diffmanifests: honour --pretty-format when printing --raw

Enable using --pretty-format to build a custom subject line
even when using the --raw option.

Change-Id: I0c1e682d984e56698fe65939aa6de12a653cd0f1
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/258565


Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Connor Newton <connor@ifthenelse.io>