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.

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).