project: fix worktree init under Windows

Git likes to create .git files with read-only permissions which makes
it difficult to open+truncate+write in situ under Windows.  Delete it
before we write the file content to workaround.

Change-Id: I3effd96525f0dfe0b90e298b6bf0b856ea26aa03
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256412
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>

project.py

@@ -3034,6 +3034,9 @@ class Project(object):
       setting = fp.read()
       assert setting.startswith('gitdir:')
       git_worktree_path = setting.split(':', 1)[1].strip()
+    # Some platforms (e.g. Windows) won't let us update dotgit in situ because
+    # of file permissions.  Delete it and recreate it from scratch to avoid.
+    platform_utils.remove(dotgit)
     # Use relative path from checkout->worktree.
     with open(dotgit, 'w') as fp:
       print('gitdir:', os.path.relpath(git_worktree_path, self.worktree),