Fix err msg for ambiguous path in git_status_file

Fix err msg for ambiguous path in git_status_file

Returning GIT_EAMBIGUOUS from inside the status callback gets
overridden with GIT_EUSER.  `git_status_file` accounted for this
via the callback payload, but was allowing the error message to
be cleared.  Move the `giterr_set` call outside the callback to
where the EUSER case was being dealt with.