Don't commit unstaged gitlinks in FREEZE.index during rebase-update.

To avoid having users deal with merge conflicts between unstaged
gitlinks modified before and after a git rebase-update we can just
not commit unstaged gitlinks in our freeze index.

This change results in whatever gitlinks are unstaged before
git rebase-update to be just rebased on top of whatever the latest
gitlinks are from remote.

Bug: 1472148

Change-Id: If7dcb82778a0bdd14c408f0a0287780a7bdc5358
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4771356
Commit-Queue: Joanna Wang <jojwang@chromium.org>
Reviewed-by: Gregory Nisbet <gregorynisbet@google.com>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
diff --git a/git_common.py b/git_common.py
index 7ff03b7..4add178 100644
--- a/git_common.py
+++ b/git_common.py
@@ -448,7 +448,7 @@
   # will be set to true if there are any indexed files to commit.
   have_indexed_files = False
 
-  for f, s in status():
+  for f, s in status(ignore_submodules='all'):
     if is_unmerged(s):
       die("Cannot freeze unmerged changes!")
     if s.lstat not in ' ?':