Fix Git base version for worktreeconfig extension

worktreeconfig extension only appears with version Git 2.20.0

Change-Id: I3ea8b7d9f8a1f7953e536edd77b09cbc4f8f3158
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/276700
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Adrien Bioteau <adrien.bioteau@gmail.com>

project.py

@@ -2793,7 +2793,7 @@ class Project(object):
 
           # Enable per-worktree config file support if possible.  This is more a
           # nice-to-have feature for users rather than a hard requirement.
-          if git_require((2, 19, 0)):
+          if git_require((2, 20, 0)):
             self.EnableRepositoryExtension('worktreeConfig')
 
       # If we have a separate directory to hold refs, initialize it as well.