Speed up 'repo start' by removing some forks

Its quite common for most projects to be matching the current
manifest revision, as most developers only modify one or two projects
at any one time.  We can speed up `repo start foo` (that impacts
the entire client) by performing most of the branch creation and
switch operations in pure Python, and thus avoid 4 forks per project.

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

project.py

@@ -30,6 +30,21 @@ from remote import Remote
 
 from git_refs import GitRefs, HEAD, R_HEADS, R_TAGS, R_PUB, R_M
 
+def _lwrite(path, content):
+  lock = '%s.lock' % path
+
+  fd = open(lock, 'wb')
+  try:
+    fd.write(content)
+  finally:
+    fd.close()
+
+  try:
+    os.rename(lock, path)
+  except OSError:
+    os.remove(lock)
+    raise
+
 def _error(fmt, *args):
   msg = fmt % args
   print >>sys.stderr, 'error: %s' % msg
@@ -758,31 +773,54 @@ class Project(object):
   def StartBranch(self, name):
     """Create a new branch off the manifest's revision.
     """
-    try:
-      self.bare_git.rev_parse(R_HEADS + name)
-      exists = True
-    except GitError:
-      exists = False;
+    head = self.work_git.GetHead()
+    if head == (R_HEADS + name):
+      return True
 
-    if exists:
-      if name == self.CurrentBranch:
-        return True
-      else:
-        cmd = ['checkout', name, '--']
-        return GitCommand(self, cmd).Wait() == 0
+    all = self.bare_ref.all
+    if (R_HEADS + name) in all:
+      cmd = ['checkout', name, '--']
+      return GitCommand(self,
+                        cmd,
+                        capture_stdout = True).Wait() == 0
 
+    branch = self.GetBranch(name)
+    branch.remote = self.GetRemote(self.remote.name)
+    branch.merge = self.revision
+
+    rev = branch.LocalMerge
+    if rev in all:
+      revid = all[rev]
+    elif IsId(rev):
+      revid = rev
     else:
-      branch = self.GetBranch(name)
-      branch.remote = self.GetRemote(self.remote.name)
-      branch.merge = self.revision
+      revid = None
 
-      rev = branch.LocalMerge
-      cmd = ['checkout', '-b', branch.name, rev]
-      if GitCommand(self, cmd).Wait() == 0:
-        branch.Save()
-        return True
-      else:
-        return False
+    if head.startswith(R_HEADS):
+      try:
+        head = all[head]
+      except KeyError:
+        head = None
+
+    if revid and head and revid == head:
+      ref = os.path.join(self.gitdir, R_HEADS + name)
+      try:
+        os.makedirs(os.path.dirname(ref))
+      except OSError:
+        pass
+      _lwrite(ref, '%s\n' % revid)
+      _lwrite(os.path.join(self.worktree, '.git', HEAD),
+              'ref: %s%s\n' % (R_HEADS, name))
+      branch.Save()
+      return True
+
+    cmd = ['checkout', '-b', branch.name, rev]
+    if GitCommand(self,
+                  cmd,
+                  capture_stdout = True).Wait() == 0:
+      branch.Save()
+      return True
+    return False
 
   def CheckoutBranch(self, name):
     """Checkout a local topic branch.