git new-branch displays an explanatory line.

BUG=

Review URL: https://codereview.chromium.org/253013004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@266905 0039d316-1c4b-4281-b951-d872f2087c98

git_new_branch.py

@@ -41,12 +41,12 @@ def main(args):
         # TODO(iannucci): Detect unclean workdir then stash+pop if we need to
         # teleport to a conflicting portion of history?
         run('checkout', '--track', opts.upstream, '-b', opts.branch_name)
-
     get_or_create_merge_base(opts.branch_name)
   except subprocess2.CalledProcessError as cpe:
     sys.stdout.write(cpe.stdout)
     sys.stderr.write(cpe.stderr)
     return 1
+  sys.stderr.write('Switched to branch %s.' % opts.branch_name)
 
 
 if __name__ == '__main__':  # pragma: no cover

subprocess2.py

@@ -510,5 +510,5 @@ def check_output(args, **kwargs):
   """
   kwargs.setdefault('stdin', VOID)
   if 'stdout' in kwargs:
-    raise ValueError('stdout argument not allowed, it will be overridden.')
+    raise ValueError('stdout argument not allowed, it would be overridden.')
   return check_call_out(args, stdout=PIPE, **kwargs)[0]