project: mark gc.log as safe to discard when migrating .git/

This is just a log file that, while useful for humans when gc aborts,
doesn't contain any data, so it's safe to throw away.

Bug: https://crbug.com/gerrit/15619
Change-Id: Ia95e0e281f52260668f7a80b5d5f990e32a8597a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/328999
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
diff --git a/project.py b/project.py
index 1225785..41e81d1 100644
--- a/project.py
+++ b/project.py
@@ -2820,7 +2820,8 @@
     }
     # Paths that we know will be in both, but are safe to clobber in .repo/projects/.
     SAFE_TO_CLOBBER = {
-        'COMMIT_EDITMSG', 'FETCH_HEAD', 'HEAD', 'gitk.cache', 'index', 'ORIG_HEAD',
+        'COMMIT_EDITMSG', 'FETCH_HEAD', 'HEAD', 'gc.log', 'gitk.cache', 'index',
+        'ORIG_HEAD',
     }
 
     # First see if we'd succeed before starting the migration.