Detect and abort when a directory was expected to be a Git working co…

…py but it isn't. One cause for this is when a directory is moved somewhere else and the old location is redefined as an svn:externals.

git-svn-clone-externals.rb

@@ -169,6 +169,7 @@ def update_current_dir
       shell('git svn fetch')
     else
       # regular update, rebase to SVN head
+      check_working_copy_git
       check_working_copy_branch
       check_working_copy_dirty
       check_working_copy_url
@@ -179,6 +180,11 @@ def update_current_dir
   end
 
 
+  def check_working_copy_git
+    raise "Error: Expected '#{Dir.getwd}' to be a Git working copy, but it isn't. Maybe a directory was replaced with an SVN externals definition. Please remove this directory and run this script again." unless File.exist?('.git')
+  end
+
+
   def check_working_copy_branch
     shell('git status')[0] =~ /On branch (\S+)/
     raise "Error: Unable to determine Git branch in '#{Dir.getwd}' using 'git status'" unless $~