Update to suggest_owners.py script

* Fixes a bunch of bugs, I should probably add an automated test.
* script now also keeps track of number of line modifications rather than
  just commits.
* Because of asking git-log for number of lines modified, it takes 10x the
  amount of time. Might make sense to add option to disable.
* script now tries to cache git-log result.
* script now ignores trivial directories that are empty except for one child
  directory.
* Fix handling of merge commits.
* Improve parsing of owners files (now understands file:// import and set
  noparent directives).
* Now also shows owners commit stats for reference.

Change-Id: I23bcf0645b3b2d330cb781ba7daa15ac9cfcb695
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1810256Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698975}

tools/git/OWNERS

 per-file move_source_file.py=satorux@chromium.org
+per-file suggest_owners.py=mheikal@chromium.org