Introduce manifest format using git submodules

If a manifest top level directory contains '.gitmodules' we now
assume this is a git module format manifest and switch to using
that code, rather than the legacy XML based manifest.

At the same time, we move the bare repository for a project from
$TOP/.repo/projects/$REPO_PATH.git to be $REPO_NAME.git instead.
This makes it easier for us to later support a repo init from an
existing work tree, as we can more accurately predict the path of
the project's repository in the workspace.  It also means that the
$TOP/.repo/projects/ directory is layed out like a mirror would be.

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/subcmds/manifest.py b/subcmds/manifest.py
index 551b13b..7a8b2ee 100644
--- a/subcmds/manifest.py
+++ b/subcmds/manifest.py
@@ -22,7 +22,7 @@
 def _doc(name):
   r = os.path.dirname(__file__)
   r = os.path.dirname(r)
-  fd = open(os.path.join(r, 'docs', 'manifest_xml.txt'))
+  fd = open(os.path.join(r, 'docs', name))
   try:
     return fd.read()
   finally:
@@ -48,6 +48,8 @@
     help = ''
     if isinstance(self.manifest, XmlManifest):
       help += self._xmlHelp + '\n' + _doc('manifest_xml.txt')
+    if isinstance(self.manifest, SubmoduleManifest):
+      help += _doc('manifest_submodule.txt')
     return help
 
   def _Options(self, p):