sync: Fix undefined variable in _FetchOne

If syncing in _FetchOne fails with GitError, sync_result does not get
set. There's already a separate local variable for success; do the same
for remote_fetched instead of referring to the conditionally defined
named tuple.

This bug is originally caused by a combination of ad8aa697 "sync: only
print error.GitError, don't raise that exception." and 1eddca84 "sync:
use namedtuples for internal return values".

Change-Id: I0f9dbafb97f8268044e5a56a6f92cf29bc23ca6a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/354176


Tested-by: Karsten Tausche <karsten@fairphone.com>
Reviewed-by: LaMont Jones <lamontjones@google.com>