project: Optimise GetCommitRevisionId when revisionId is set

When comparing 2 manifests, most of the time is
spent getting the relevant commit id as it relies
on _allrefs which ends up loading all git references.

However, the value from `revisionIs` (when it is valid)
could be used directly leading to a huge performance improvement
(from 180+ seconds to less than 0.01 sec which is more
than 25000 times faster for manifests with 700+ projects).

Bug: 295282548

Change-Id: I5881aa4b2326cc17bbb4ee91d23293111f76ad7e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/385834
Tested-by: Sylvain Desodt <sylvain.desodt@gmail.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Sylvain Desodt <sylvain.desodt@gmail.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/project.py b/project.py
index 34e9046d..f2f81d7a 100644
--- a/project.py
+++ b/project.py
@@ -1443,6 +1443,8 @@ class Project(object):
         rather than the id of the current git object (for example, a tag)
 
         """
+        if self.revisionId:
+            return self.revisionId
         if not self.revisionExpr.startswith(R_TAGS):
             return self.GetRevisionId(self._allrefs)