Introduce git merge driver for the blink reformatting

gclient.py

Index: gclient.py
diff --git a/gclient.py b/gclient.py
index 88edbd7af489b3034a16a2c4b3cae426af2f5908..8efa5d1a8dfd7cad49fa40022ea857ac7c0ecf89 100755
--- a/gclient.py
+++ b/gclient.py
@@ -883,6 +883,28 @@ class Dependency(gclient_utils.WorkItem, DependencySettings):
 
     # Always parse the DEPS file.
     self.ParseDepsFile()
+
+    # Setup the merge driver for The Great Blink Reformatting. This does just
+    # setup a custom merge-driver in the git config of the Chromium main project
+    # but does not have any effect until somebody changes the .gitattributes
+    # to refer to it. See crbug.com/574611 for more.
+    # TODO(primiano): remove some weeks after the reformatting. ETA: Oct 2016.
+    if parsed_url == CHROMIUM_SRC_URL:
+      CFG_PREFIX = 'merge.clang_format_merge_driver'

[Nico, 2016/09/20 21:54:26]

I thought I had seen a way to manually pass this to `git rebase`, like `git
rebase --tool=clang-format` but I can't find that flag now that I try to find
it. If there's a flag like that, giving this a shorter name might be nice.

[dcheng, 2016/09/20 22:11:19]

I think it's:
git mergetool --tool=clang_format_merge_driver

I apparently come from the Java school of naming though, so I'm having trouble
thinking of a better name that's descriptive here. How does clang_format_merger
sound?

[Nico, 2016/09/20 22:26:48]

git mergetool --tool=reformat
git mergetool --tool=clang-format
git mergetool --tool=reformat-merge
git mergetool --tool=clang-format-merge
git mergetool --tool=clang_format_merger

Hm, not sure which one I like best. I'd personally prefer - over _, and I'd err
on the side of brevity -- I'd go with either "reformat" or
"clang-format-merge"...maybe the latter. But in the end it's a bikeshed and
you're writing the CL, so you get to pick the final color :-)

[dcheng, 2016/09/20 22:30:47]

Oh right, we can still keep the script in clang_format_merge_driver.

I like clang-format best, assuming it works (it's short, and it's implicitly a
mergetool, due to where it lives in the config anyway)

+      try:
+        merge_driver = self._used_scm._Capture(
+            ['config', CFG_PREFIX + '.driver']).strip()
+      except subprocess2.CalledProcessError:
+        merge_driver = None
+
+      if not merge_driver:
+        print('Installing clang-format merge driver for crbug/574611')
+        self._used_scm._Capture(['config', CFG_PREFIX + '.name',
+                                 'clang-format merge driver (crbug/574611)'])
+        self._used_scm._Capture(['config', CFG_PREFIX + '.driver',
+                                 'clang_format_merge_driver %O %A %B %P'])
+        self._used_scm._Capture(['config', CFG_PREFIX + '.recursive', 'binary'])

[Nico, 2016/09/20 21:54:26]

do we need to set trustExitCode here too, or is that set elsewhere?

[dcheng, 2016/09/20 22:11:19]

I'm not sure, honestly. git is magic. It seems to work automatically already
without it?

+
     self._run_is_done(file_list or [], parsed_url)
     if command in ('update', 'revert') and not options.noprehooks:
       self.RunPreDepsHooks()