dim: autodetect branches in rebuild-nightly

Needs new url-mapping information in nightly.conf to work properly. Only
leftover bit from my original patch after rebasing is to abstract away
the remote for the integration tree.

v2:
- Fix typo when calling rr_cache_dir. Oops.
- Use cut -f 1 instead of sed/regex horrors (jani).

v3: Fix bug that resulted in nesting rr-cache directories.

v4: Rebasing.

v5: Resurrect $repo (as $remote) printing when fetching branches,
originally from my patch.

v6: Use remote instead of origin local variables, to be more
consistent. Picked up from Jani.

v7: Move misplaced hunks to right place.

v8: For the nightly remote use nightly_remote instead of anything
called origin - it's confusing.

v9: Create a separate rerere_remote for the rerere-cache branch. It
might be in a difference local git repository (Jani).

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>