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>