This CL provides minimal `git cl format` support to enforce correct Rust
formatting in presubmit checks.  For now the files are always fully
formatted - there is no support at this point for formatting only the
changed lines.

Manual tests (after temporarily, artificially introducing a formatting
error to one of .rs files under build/rest/tests):

*) git cl presubmit
   Result: The src directory requires source formatting.
           Please run: git cl format

*) git cl format --dry-run
   Result: Pretty/colorful diff printed out.

*) git cl format
   Result: Temporary formatting errors are fixed.

Bug: chromium:1231317
Change-Id: I114ece90630476f27871ebcd170162caa92c0871
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3054980
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Adrian Taylor <adetaylor@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: danakj <danakj@chromium.org>