deps2git: Add support for overriding DEPS vars

deps2git.py

Index: deps2git.py
===================================================================
--- deps2git.py	(revision 292133)
+++ deps2git.py	(working copy)
@@ -280,6 +280,11 @@
   return results
 
 
+def convert_json(option, _, value, parser):
+  """Provide an OptionParser callback to unmarshal a JSON string."""
+  setattr(parser.values, option.dest, json.loads(value))
+
+
 def main():
   parser = optparse.OptionParser()
   parser.add_option('-d', '--deps', default='DEPS',
@@ -300,6 +305,10 @@
                     help='top level of a gclient git cache diretory.')
   parser.add_option('-s', '--shallow', action='store_true',
                     help='Use shallow checkouts when populating cache dirs.')
+  parser.add_option('--var_overrides', action='callback', callback=convert_json,
+                    type='string', default=None,
+                    help='A dict in JSON format containing key-value pairs '
+                         'that will override variables used in the DEPS file.')
   parser.add_option('--no_fail_fast', action='store_true',
                     help='Try to process the whole DEPS, rather than failing '
                     'on the first bad entry.')
@@ -311,7 +320,7 @@
 
   # Get the content of the DEPS file.
   deps, deps_os, include_rules, skip_child_includes, hooks, deps_vars = (
-      deps_utils.GetDepsContent(options.deps))
+      deps_utils.GetDepsContent(options.deps, options.var_overrides))
 
   if options.extra_rules and options.type:
     parser.error('Can\'t specify type and extra-rules at the same time.')