Slightly simplify LastCommitCache (#20444)

The LastCommitCache code is a little complex and there is unnecessary
duplication between the gogit and nogogit variants.

This PR adds the LastCommitCache as a field to the git.Repository and
pre-creates it in the ReferencesGit helpers etc. There has been some
simplification and unification of the variant code.

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/git/commit.go

@@ -80,6 +80,9 @@ func (c *Commit) ParentCount() int {
 
 // GetCommitByPath return the commit of relative path object.
 func (c *Commit) GetCommitByPath(relpath string) (*Commit, error) {
+	if c.repo.LastCommitCache != nil {
+		return c.repo.LastCommitCache.GetCommitByPath(c.ID.String(), relpath)
+	}
 	return c.repo.getCommitByPathWithID(c.ID, relpath)
 }