Teach Project how to relink a .git/ in the work tree

The _LinkWorkTree method can now be used to relink the work tree,
such as if the real repository was moved to a different location
on disk.

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

project.py

@@ -1102,9 +1102,9 @@ class Project(object):
         msg = 'manifest set to %s' % self.revisionExpr
         self.bare_git.symbolic_ref('-m', msg, ref, dst)
 
-  def _InitWorkTree(self):
+  def _LinkWorkTree(self, relink=False):
     dotgit = os.path.join(self.worktree, '.git')
-    if not os.path.exists(dotgit):
+    if not relink:
       os.makedirs(dotgit)
 
     for name in ['config',
@@ -1120,6 +1120,8 @@ class Project(object):
       try:
         src = os.path.join(self.gitdir, name)
         dst = os.path.join(dotgit, name)
+        if relink:
+          os.remove(dst)
         os.symlink(relpath(src, dst), dst)
       except OSError, e:
         if e.errno == errno.EPERM:
@@ -1127,6 +1129,11 @@ class Project(object):
         else:
           raise
 
+  def _InitWorkTree(self):
+    dotgit = os.path.join(self.worktree, '.git')
+    if not os.path.exists(dotgit):
+      self._LinkWorkTree()
+
       _lwrite(os.path.join(dotgit, HEAD), '%s\n' % self.GetRevisionId())
 
       cmd = ['read-tree', '--reset', '-u']