Keep old git alternatives when changing mirrors

It's possible to change DEPS git repository while keeping the same
source. This is useful feature, for example if upstream repository moves
from github to gitlab. The repositories can be out of sync, or even
diverge.

We use git alternatives to save disk space. If a builder flip-flops
between mirror repositories (e.g. case with different release branches),
it's possible that new mirror doesn't contain all necessary objects that
are present in the old repositories. That will result in invalid git
pointer entries and fetch operations will fail.

Instead of replacing git alternatives, we append new git alternatives to
avoid such problem.

R=bpastene@google.com, jojwang@google.com

Bug: b/241802975
Change-Id: I3ef025603af58a84692a68bf3b780c47b6d3ae46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3843133
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

gclient_scm.py

@@ -697,10 +697,15 @@ class GitWrapper(SCMWrapper):
       # Switch over to the new upstream
       self._Run(['remote', 'set-url', self.remote, url], options)
       if mirror:
+        # Because we use Git alternatives, our existing repository is not
+        # self-contained. It's possible that new git alternative doesn't have
+        # all necessary objects that the current repository needs. Instead of
+        # blindly hoping that new alternative contains all necessary objects,
+        # keep the old alternative and just append a new one on top of it.
         with open(os.path.join(
             self.checkout_path, '.git', 'objects', 'info', 'alternates'),
-            'w') as fh:
-          fh.write(os.path.join(url, 'objects'))
+            'a') as fh:
+          fh.write("\n" + os.path.join(url, 'objects'))
       self._EnsureValidHeadObjectOrCheckout(revision, options, url)
       self._FetchAndReset(revision, file_list, options)