And merge it in use_relative_paths. In all Chromium repos,
use_relative_hooks is True iff:

 - use_relative_paths is True, and
 - there is at least one hook

It makes sense that you'd want to use relative hooks if you use relative
paths so this CL merges both flags into use_relative_paths.

Bug: chromium:1107325
Change-Id: I0fe40eba1e1c61be26c812c7ca4329efb72c7f90
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2306795
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Auto-Submit: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>