project: fix diff printing with embedded %

The recent commit 84230009 ("project:
make diff tools synchronous") broke repo diff if it includes % formats.
Add an explicit format string to fix.

Bug: https://crbug.com/gerrit/14208
Change-Id: Ie255a43c5b767488616b2b3dd15abc18f93bfab2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/299402


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>