Note: This feature was developed independently on master while this was
in review. I've removed the conflicting bits and left the relevant
additions, mainly a test for `Gitlab::Git::Hook`. The original commit
message follows:

1. `gitlab-shell` outputs errors to `stderr`, but we weren't using this
   information, prior to this commit. Now we capture the `stderr`, and
   display it in the flash message when branch creation fails.

2. This can be used to display better errors for other git operation
   failures with small tweaks.

3. The return value of `Gitlab::Git::Hook#trigger` is changed from a
   simple `true`/`false` to a tuple of `[status, errors]`. All usages
   and tests have been updated to reflect this change.

4. This is only relevant to branch creation _from the Web UI_, since SSH
   and HTTP pushes access `gitlab-shell` either directly or through
   `gitlab-workhorse`.

5. A few minor changes need to be made on the `gitlab-shell` end. Right
   now, the `stderr` message it outputs is prefixed by "GitLab: ", which
   shows up in our flash message. This is better removed.