This fixes Gerrit project detection based on remote URL,
accounting for potential 'a/' prefix in the URL path component,
which isn't part of the Gerrit project name.

R=ehmaldonado, mmoss

Bug: 876964, 876910
Change-Id: I473ae8c6c9e0f2034b350901abd67db151e0a3d3
Reviewed-on: https://chromium-review.googlesource.com/1187573Reviewed-by: Michael Moss <mmoss@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>