Parses git diff for changed python files in a method similar to
clang-diff and feeds the resulting line ranges into yapf.

Also sets the default style to the yapf file included in depot tools
by searching parent directories of each changed file to find a yapf
style config (.style.yapf). If none is found the default style
file will be the chromium .style.yapf included in depot tools.

Note: Even if line ranges are specified, yapf will fix indentation
issues for the entire file.
This is intended see https://github.com/google/yapf/issues/499
This may cause some issues if git cl format is run on a file
with lots of indentation issues or on a file or when run on a
third_party file that is formatted with pep8 and does not include
a .style.yapf and may make many more changes then the user expects.

Still undecided on whether this should be turned on by default but
if not I think the non --full support is a positive change anyways.

Bug:846432
Change-Id: Ib85797f4a8e1021870901ff465ec10f7e70deb87
Reviewed-on: https://chromium-review.googlesource.com/1249642
Commit-Queue: Aiden Benner <abenner@google.com>
Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>