sync: Recover from errors during read-tree

When repo is initializing a git repository, it calls `git read-tree`.
During such operation, git is restoring workspace based on the current
index. However, some things can go wrong: a user can run out of disk
space, or, in case of partial clone, user may no longer reach the remote
host. That will leave affected repository in a bad state with partially
checked out workspace. The follow up repo sync won't try to fix such
state.

This change removes .git symlink, which will force the next `repo sync`
to redo Git repository setup.

Bug: b/363171216
Bug: b/390161127
Change-Id: I57db4b6cae0ef21826dc7cede4d3bf02cfc3d955
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/447801


Reviewed-by: Scott Lee <ddoman@google.com>
Tested-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>