Teach repo how to download changes to the local checkout

Now `repo download . 1402` would download the change numbered 1402
into the current project and check it out for the user, using a
detached HEAD.  `repo sync .` would back out of the change and
return to the upstream version.

Multiple projects can be fetched at once by listing them out on
the command line as different arguments.

Individual patch sets can be selected by adding a '/n' to indicate
the n-th patch set should be downloaded instead of the default of
patch set 1.

Signed-off-by: Shawn O. Pearce <sop@google.com>

project.py

@@ -45,6 +45,31 @@ def _info(fmt, *args):
 def not_rev(r):
   return '^' + r
 
+class DownloadedChange(object):
+  _commit_cache = None
+
+  def __init__(self, project, base, change_id, ps_id, commit):
+    self.project = project
+    self.base = base
+    self.change_id = change_id
+    self.ps_id = ps_id
+    self.commit = commit
+
+  @property
+  def commits(self):
+    if self._commit_cache is None:
+      self._commit_cache = self.project.bare_git.rev_list(
+        '--abbrev=8',
+        '--abbrev-commit',
+        '--pretty=oneline',
+        '--reverse',
+        '--date-order',
+        not_rev(self.base),
+        self.commit,
+        '--')
+    return self._commit_cache
+
+
 class ReviewableBranch(object):
   _commit_cache = None
 
@@ -612,6 +637,23 @@ class Project(object):
     src = os.path.join(self.worktree, src)
     self.copyfiles.append(_CopyFile(src, dest))
 
+  def DownloadPatchSet(self, change_id, patch_id):
+    """Download a single patch set of a single change to FETCH_HEAD.
+    """
+    remote = self.GetRemote(self.remote.name)
+
+    cmd = ['fetch', remote.name]
+    cmd.append('refs/changes/%2.2d/%d/%d' \
+               % (change_id % 100, change_id, patch_id))
+    cmd.extend(map(lambda x: str(x), remote.fetch))
+    if GitCommand(self, cmd, bare=True).Wait() != 0:
+      return None
+    return DownloadedChange(self,
+                            remote.ToLocal(self.revision),
+                            change_id,
+                            patch_id,
+                            self.bare_git.rev_parse('FETCH_HEAD'))
+
 
 ## Branch Management ##