cbuildbot: allow branches to track locally.
cbuildbot: allow branches to track locally.

This artifical restriction has been the source of multiple lost
patches/changes in my workflow- I directly use tracking branches
that locally track, yet cbuildbot refuses to handle it.  The end
result of that is that if I've got 4 branches that are dependant
on one feature, I have to carefully go through each making sure
I've not inadvertantly changed the dependant branch (this is
exactly what rebase is good for; easing separation of changes).

In particular, CL I9b042c05 I had a *lot* of issues with this due
to each step needing work/stabilization, but still needing to
finalize/debug later patches in the series; while that CL had
it's own issues, definitely can trace some of the issues back to
the fact fixes/changes had to be transfered across multiple branches
when it should've just lived in a single tracking branch.

An example of a local tracking is thus:

git branch manifest-cleanups -t remotes/m/master
git branch cbuildbot-ui -t manifest-cleanups

In that case, one would be segrating the changes into the appropriate
pool; in doing it this way, it makes it *far* easier to ensure that
changes do not get lost when you've got multiple features dependant
on a specific feature branch.

This CL changes the git tracking lookup code such that it defaults to
recursing if the remote is '.' (controllable, but defaults to
recursing).  Effectively, we do the 'right' thing and trace it back
to a non local remote, preserving the intent while removing the
artificial restriction.

BUG=None
TEST=setup branches as described above, checking out cbuildbot-ui,
     then `cbuildbot x86-generic-full -p chromiumos/chromite`.  Previously,
     this would immediately fail; now it doesn't.

Change-Id: I0746fc0284863b5a7d0a847fc92f9feea7d03eb9
Reviewed-on: https://gerrit.chromium.org/gerrit/22652
Reviewed-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Tested-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Commit-Ready: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>

zhang0137/chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,zhang0137/chromite,coreos/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite