git_utils.py: allow checkouts of local repositories at any commithash along master branch

py/utils/git_utils.py

Index: py/utils/git_utils.py
diff --git a/py/utils/git_utils.py b/py/utils/git_utils.py
index 52ad83a6409fd4ff17853e966ba69b68775cdc28..9abde2e1506b3773c47522fb9e279046ad745fbd 100644
--- a/py/utils/git_utils.py
+++ b/py/utils/git_utils.py
@@ -152,8 +152,8 @@ class GitBranch(object):
 class NewGitCheckout(object):
   """Creates a new local checkout of a Git repository."""
 
-  def __init__(self, repository, refspec=None, subdir=None,
-               containing_dir=None):
+  def __init__(self, repository, branch='master', commithash='HEAD',
+               subdir=None, containing_dir=None):
     """Check out a new local copy of the repository.
 
     Because this is a new checkout, rather than a reference to an existing
@@ -168,9 +168,12 @@ class NewGitCheckout(object):
     # the checkout is automatically cleaned up here
 
     Args:
-      repository: name of the remote repository
-      refspec: an arbitrary remote ref (e.g., the name of a branch);
-          if None, allow the git command to pick a default
+      repository: URL of the remote repository (e.g.,
+          'https://skia.googlesource.com/common') or path to a local repository
+          (e.g., '/path/to/repo/.git') to check out a copy of
+      branch: which branch to fetch from the repository
+      commithash: rewind the local checkout to a particular commithash (which
+          must be present along the branch specified above)
       subdir: if specified, the caller only wants access to files within this
           subdir in the repository.
           For now, we check out the entire repository regardless of this param,
@@ -188,11 +191,13 @@ class NewGitCheckout(object):
     else:
       self._file_root = self._git_root
 
-    pull_cmd = [GIT, 'pull', repository]
-    if refspec:
-      pull_cmd.append(refspec)
+    local_branch_name = 'local'
     self._run_in_git_root(args=[GIT, 'init'])
-    self._run_in_git_root(args=pull_cmd)
+    self._run_in_git_root(args=[GIT, 'fetch', repository, branch])
+    self._run_in_git_root(args=[GIT, 'merge', 'FETCH_HEAD'])
+    self._run_in_git_root(args=[GIT, 'branch', local_branch_name, commithash])
+    self._run_in_git_root(args=[GIT, 'checkout', local_branch_name])

[borenet, 2014/08/19 14:30:31]

Why not just do:
$ git clone <repository>
$ git checkout -b <branch> -t <commithash|upstream_branch>
?

[epoger, 2014/08/19 15:16:03]

Seems to work (with some adjustments).  I think the new way is less efficient
for large remote repositories, because "git clone" fetches the history for all
advertised branches.  But if you prefer it this way, it's fine with me.

[borenet, 2014/08/19 15:24:28]

What repository are we checking out?  I think Skia's repo is small enough that
it shouldn't make much difference.  In fact, since everything gets pushed onto
master, the only thing we'd miss are the Chrome release branches.

[epoger, 2014/08/19 15:42:05]

The primary use case is indeed the Skia repo.  Personally, I think it would be
better to call the component steps rather than "git clone", for efficient
operation if someone *does* use this to check out from a large repo.  But I'm
happy either way.

[borenet, 2014/08/19 15:50:20]

SGTM.  You're welcome to change it back if you want.

+
 
   @property
   def root(self):