git cl upload: also explicitly use eu1 mirror for retries.

R=ehmaldonado

Bug: 881860
Change-Id: Ia02905a8b71486d771a691c31114d692e62a369f
Reviewed-on: https://chromium-review.googlesource.com/c/1277796Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

gerrit_util.py

@@ -982,7 +982,7 @@ def ChangeIdentifier(project, change_number):
 
 
 # TODO(crbug/881860): remove this hack.
-_GERRIT_MIRROR_PREFIXES = ['us1', 'us2', 'us3']
+_GERRIT_MIRROR_PREFIXES = ['us1', 'us2', 'us3', 'eu1']
 assert all(3 == len(p) for p in _GERRIT_MIRROR_PREFIXES)