On a new Linux box, during git cl dcommit I observed
the error:

  fatal: invalid diff option/value: -r

This was caused by the git cl presubmit hooks running:

  git diff --name-status -r refs/remotes/origin/trunk.. /path/to/src

By passing a relative path (given by git rev-parse
--show-cdup) instead of an absolute path for
/path/to/src, git diff --name-status does not print an
error and returns the expected list of changed files.

Also fix some broken scm and gclient_scm unit tests.

BUG=none
TEST=git presubmit hooks work from main working
directory
Review URL: http://codereview.chromium.org/438012

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@32999 0039d316-1c4b-4281-b951-d872f2087c98