[fetch] Set diff.ignoreSubmodules to dirty

none is too slow for chromium/src (4x), and all doesn't work well with
git submodules.

R=jojwang

Bug: 1470195
Change-Id: I8fd4e27e68ff36b14c9f531a1f90b8d76b8f55d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4802510
Auto-Submit: Josip Sokcevic <sokcevic@chromium.org>
Reviewed-by: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Joanna Wang <jojwang@chromium.org>

fetch.py

@@ -158,7 +158,7 @@ class GclientGitCheckout(GclientCheckout, GitCheckout):
       self.run_git(
           'config', '--add', 'remote.origin.fetch',
           '+refs/tags/*:refs/tags/*', cwd=wd)
-    self.run_git('config', 'diff.ignoreSubmodules', 'all', cwd=wd)
+    self.run_git('config', 'diff.ignoreSubmodules', 'dirty', cwd=wd)
 
 
 CHECKOUT_TYPE_MAP = {