Properly handle git branches with slashes in tryclient.

Buildbot try --vc=git breaks when branches contain slashes. This patch fixes this by stripping only proper (i.e configured in current git repo) remote names.

Signed-off-by: David Gstir <david@sigma-star.at>
Merge sigma-star/buildbot:tryclient-git-branches-fix (PR #1158)