Revert "[depot_tools] Add --fork-point when computing merge-base"

This reverts commit 934ac6ebdf6bae93a83c75d4eccd36e7dbfaa13d.

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>
diff --git a/git_common.py b/git_common.py
index 8008c62..54e65e8 100644
--- a/git_common.py
+++ b/git_common.py
@@ -530,11 +530,7 @@
   parent = parent or upstream(branch)
   if parent is None or branch is None:
     return None
-
-  try:
-    actual_merge_base = run('merge-base', '--fork-point', parent, branch)
-  except subprocess2.CalledProcessError:
-    actual_merge_base = run('merge-base', parent, branch)
+  actual_merge_base = run('merge-base', parent, branch)
 
   if base_upstream != parent:
     base = None