A few bugs slipped in at the last moment which broke posting changes in ClearCase repositories when using the legacy community-driven ClearCase backend on the server. This change fixes them up: - The return for the diff lines got indented one space too far, which meant we were never returning it for the legacy case. This resulted in always getting a "there don't seem to be any diffs" error. - The backend has `diffs_use_absolute_paths=False` (by virtue of not changing the default). This means that it's expected to send a basedir along with any diffs. The old implementation of the repository info structure set the `base_path` to be the vob tag, but this got lost when we redid everything. Since we only need this for the legacy implementation, I've made it so we set base_path after we negotiate with the server about which backend we're talking to. - The filenames listed in the diff header used to contain the vob tag. This of course duplicates information which is already present in both the server-side configuration of the repository AND the basedir, but not including it in the filenames causes the diff parser to hit an infinite loop. Testing Done: Posted changes against a repository configured with the legacy backend (along with some changes to that backend which will be coming in a separate review request).