worktree: Do not try to fix relative paths

--worktree was broken with incorrect paths in the .git files
whenever the local copy of git populated gitdir with relative paths
instead of absoulte paths.

Bug: 376251410
Change-Id: Id32dc1576315218967de2a9bfe43bf7a5a0e7aa6
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/440801


Commit-Queue: Allen Webb <allenwebb@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Tested-by: Allen Webb <allenwebb@google.com>