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>

git_config.py

@@ -57,6 +57,7 @@ class GitConfig(object):
     self.file = file
     self.defaults = defaults
     self._cache_dict = None
+    self._section_dict = None
     self._remotes = {}
     self._branches = {}
     self._pickle = os.path.join(
@@ -168,6 +169,33 @@ class GitConfig(object):
       self._branches[b.name] = b
     return b
 
+  def HasSection(self, section, subsection = ''):
+    """Does at least one key in section.subsection exist?
+    """
+    try:
+      return subsection in self._sections[section]
+    except KeyError:
+      return False
+
+  @property
+  def _sections(self):
+    d = self._section_dict
+    if d is None:
+      d = {}
+      for name in self._cache.keys():
+        p = name.split('.')
+        if 2 == len(p):
+          section = p[0]
+          subsect = ''
+        else:
+          section = p[0]
+          subsect = '.'.join(p[1:-1])
+        if section not in d:
+          d[section] = set()
+        d[section].add(subsect)
+        self._section_dict = d
+    return d
+
   @property
   def _cache(self):
     if self._cache_dict is None:
@@ -443,11 +471,23 @@ class Branch(object):
   def Save(self):
     """Save this branch back into the configuration.
     """
-    self._Set('merge', self.merge)
-    if self.remote:
-      self._Set('remote', self.remote.name)
+    if self._config.HasSection('branch', self.name):
+      if self.remote:
+        self._Set('remote', self.remote.name)
+      else:
+        self._Set('remote', None)
+      self._Set('merge', self.merge)
+
     else:
-      self._Set('remote', None)
+      fd = open(self._config.file, 'ab')
+      try:
+        fd.write('[branch "%s"]\n' % self.name)
+        if self.remote:
+          fd.write('\tremote = %s\n' % self.remote.name)
+        if self.merge:
+          fd.write('\tmerge = %s\n' % self.merge)
+      finally:
+        fd.close()
 
   def _Set(self, key, value):
     key = 'branch.%s.%s' % (self.name, key)