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

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>

git_common.py

@@ -530,11 +530,7 @@ def get_or_create_merge_base(branch, parent=None):
   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

tests/git_common_test.py

@@ -693,36 +693,6 @@ class GitMutableStructuredTest(git_test_utils.GitRepoReadWriteTestBase,
     self.assertIsNone(
       self.repo.run(self.gc.get_or_create_merge_base, 'branch_DOG'))
 
-  def testMergeBaseWithForkPoint(self):
-    self.repo.git('commit', '--allow-empty', '-am', 'foooooo')
-    self.repo.git('checkout','-tb', 'foobarA', 'main')
-    foobarA = self.repo.run(self.gc.hash_one, 'foobarA', short=True)
-
-    self.repo.git('checkout','-tb', 'foobarB', 'foobarA')
-    with self.repo.open('cl1', 'w') as f:
-        f.write('cl1')
-    self.repo.git('add', 'cl1')
-    self.repo.git_commit('cl1')
-    foobarB_old = self.repo.run(self.gc.hash_one, 'foobarB', short=True)
-
-    self.repo.git('checkout','-tb', 'foobarC', 'foobarB')
-    with self.repo.open('cl2', 'w') as f:
-        f.write('cl2')
-    self.repo.git('add', 'cl2')
-    self.repo.git_commit('cl2')
-    foobarC = self.repo.run(self.gc.hash_one, 'foobarC', short=True)
-
-    self.repo.git('checkout', 'foobarB')
-    with self.repo.open('cl1', 'w') as f:
-      f.write('amend cl1')
-    self.repo.git('add', 'cl1')
-    self.repo.git('commit', '--amend', '-m', 'amend cl1')
-
-    self.assertIn(foobarA,
-      self.repo.run(self.gc.get_or_create_merge_base, 'foobarB', foobarB_old))
-    self.assertIn(foobarB_old,
-      self.repo.run(self.gc.get_or_create_merge_base, foobarC, 'foobarB'))
-
   def testGetBranchTree(self):
     skipped, tree = self.repo.run(self.gc.get_branch_tree)
     # This check fails with git 2.4 (see crbug.com/487172)