Implement git-drover.

git_common.py

Index: git_common.py
diff --git a/git_common.py b/git_common.py
index 6952c1ee318765ebe40622ec9a7b5d10334c888f..2266502838f11080ec6eb968edecafc0e14f538a 100644
--- a/git_common.py
+++ b/git_common.py
@@ -30,6 +30,8 @@ import threading
 
 import subprocess2
 
+import git_cache
+
 
 GIT_EXE = 'git.bat' if sys.platform.startswith('win') else 'git'
 TEST_MODE = False
@@ -215,9 +217,10 @@ def once(function):
   """@Decorates |function| so that it only performs its action once, no matter
   how many times the decorated |function| is called."""
   def _inner_gen():
-    yield function()
+    ret = function()
+    yield ret
     while True:
-      yield
+      yield ret
   return _inner_gen().next
 
 
@@ -275,6 +278,23 @@ def run_with_retcode(*cmd, **kwargs):
     return cpe.returncode
 
 
+def cached_fetch(commits):
+  m = git_cache.Mirror.from_repo('.')
+  if m:
+    m.populate(fetch_specs=commits)
+  else:
+    for commit in commits:
+      run('fetch', 'origin', commit)
+
+
+def check(*args, **kwargs):
+  try:
+    run(*args, **kwargs)
+    return True
+  except subprocess2.CalledProcessError:
+    return False
+
+
 def config(option, default=None):
   try:
     return run('config', '--get', option) or default
@@ -375,6 +395,14 @@ def get_or_create_merge_base(branch, parent=None):
   return base
 
 
+def get_remote_url(remote):
+  m = git_cache.Mirror.from_repo('.')
+  if m:
+    return m.url
+
+  return config('remote.%s.url' % remote)
+
+
 def hash_multi(*reflike):
   return run('rev-parse', *reflike).splitlines()
 
@@ -512,9 +540,12 @@ def run_with_stderr(*cmd, **kwargs):
   kwargs.setdefault('stdout', subprocess2.PIPE)
   kwargs.setdefault('stderr', subprocess2.PIPE)
   autostrip = kwargs.pop('autostrip', True)
+  verbose = kwargs.pop('verbose', False)
   indata = kwargs.pop('indata', None)
 
   cmd = (GIT_EXE, '-c', 'color.ui=never') + cmd
+  if verbose:
+    print "running `%s`" % (cmd,)
   proc = subprocess2.Popen(cmd, **kwargs)
   ret, err = proc.communicate(indata)
   retcode = proc.wait()
@@ -657,3 +688,7 @@ def upstream(branch):
                branch+'@{upstream}')
   except subprocess2.CalledProcessError:
     return None
+
+
+def verify_commit(commit):
+  return check('rev-parse', '--quiet', '--verify', '%s^{commit}' % commit)