Fix infinite loop when FindRepoDir argument is a relative path.

This bug doesn't affect production but affects the unit tests.

BUG=chromium-os:9201
TEST=Confirmed this fixes an infinite loop in the unit tests, and that
    cbuildbot.py still works.

Change-Id: I1a56b92d8229110c84cebbe877e55fb99f053212

Review URL: http://codereview.chromium.org/5025002

lib/cros_build_lib.py

@@ -146,6 +146,7 @@ def FindRepoDir(path=None):
   """
   if path is None:
     path = os.getcwd()
+  path = os.path.abspath(path)
   while path != '/':
     repo_dir = os.path.join(path, '.repo')
     if os.path.isdir(repo_dir):