This reverts commit 934ac6eb.

Reason for revert: https://crbug.com/1225658, git reparent-branch
may result in data loss (recoverable if gc doesn't remove it).

Original change's description:
> [depot_tools] Add --fork-point when computing merge-base
>
> Using --fork-point when possible reduces the number of merge conflicts when running merge-base.
>
> Bug: 1111269
> Change-Id: I7240fdc9a613d2eb4e856b5677fc713551d7afe9
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2519729
> Reviewed-by: Josip Sokcevic <sokcevic@google.com>
> Commit-Queue: Gavin Mak <gavinmak@google.com>

R=gavinmak@google.com

Bug: 1111269
Bug: 1225658
Change-Id: I2aeadeb2fbff18abcab5552e07254dfc1c2499a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3034831
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>