Re-reland r245404 ("If the destination directory doesn't contain the desired repo, delete it")

scm.py

Index: scm.py
diff --git a/scm.py b/scm.py
index 30bb3d17ea49bd74c0a83af490bbc486d60e3f41..60ba80000990b50615c366d543bfab2c547b5ac2 100644
--- a/scm.py
+++ b/scm.py
@@ -166,12 +166,15 @@ class GIT(object):
     return GIT.ShortBranchName(GIT.GetBranchRef(cwd))
 
   @staticmethod
-  def IsGitSvn(cwd):
+  def IsGitSvn(checkout_root):
     """Returns true if this repo looks like it's using git-svn."""
+    # A git-svn checkout has a .git directory.
+    if not os.path.isdir(os.path.join(checkout_root, '.git')):
+      return False
     # If you have any "svn-remote.*" config keys, we think you're using svn.
     try:
       GIT.Capture(['config', '--local', '--get-regexp', r'^svn-remote\.'],
-                  cwd=cwd)
+                  cwd=checkout_root)
       return True
     except subprocess2.CalledProcessError:
       return False
@@ -408,7 +411,7 @@ class GIT(object):
   @staticmethod
   def GetSha1ForSvnRev(cwd, rev):
     """Returns a corresponding git sha1 for a SVN revision."""
-    if not GIT.IsGitSvn(cwd=cwd):
+    if not GIT.IsGitSvn(cwd):
       return None
     try:
       output = GIT.Capture(['svn', 'find-rev', 'r' + str(rev)], cwd=cwd)