This is a reland of e3a49aa4

GetRelativeRoot was fixed to use '.' instead of None

Original change's description:
> git-cl: Remove unused and duplicate functions.
>
> Remove unused functions, and use scm.GIT where possible to reduce
> duplication.
>
> Change-Id: I24f05e7b3ae04743e97b6665ee668f44d6acc294
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2116938
> Reviewed-by: Anthony Polito <apolito@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

Change-Id: I54bd1d34d3501a38bf8af2f9e70059d58fa538a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2125293Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>