project: fix diff printing with embedded %

The recent commit 84230009ee4282b947482f0d4fc4fe9e9ebc9e01 ("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>
diff --git a/project.py b/project.py
index 474d657..45b6c63 100644
--- a/project.py
+++ b/project.py
@@ -863,7 +863,7 @@
       out.nl()
       out.project('project %s/' % self.relpath)
       out.nl()
-      out.write(p.stdout)
+      out.write('%s', p.stdout)
     return p.Wait() == 0
 
 # Publish / Upload ##