Minor build-time patching usability improvements.

* Avoid an unnecessary |repo abandon| if no build-time patches exist
* Reduce stdout messages
* Add support for *.sha1 patch files, which can be used to cherry-pick commits
  from other branches of the given project.  File format is the output of
  |git log --oneline|

Change-Id: Ia40895f8d30fb5d46be4fef30a7f99142aa07bf9