init: reset hard to --repo-rev

When updating the tracking ref to whatever the user requested,
make sure we reset state completely rather than trying to update
the ref to it.  This avoids confusing git as to the current state
of the tree, and is more inline with user intentions: if they made
a local change to the checkout, but ran repo init with a specific
rev, we shouldn't stay wedged forever until they manually clean it
all up.

Bug: https://crbug.com/gerrit/12801
Change-Id: Ieba8d9c15781b4d0649bf01c7460694da63387b2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/290923
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

subcmds/init.py

@@ -521,7 +521,7 @@ to update the working directory files.
           rp.gitdir, opt.repo_rev, repo_verify=opt.repo_verify, quiet=opt.quiet)
       branch = rp.GetBranch('default')
       branch.merge = remote_ref
-      rp.work_git.update_ref('refs/heads/default', rev)
+      rp.work_git.reset('--hard', rev)
       branch.Save()
 
     if opt.worktree: