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>