remote-hg: unquote C-style paths when exporting

git-fast-import documentation says that paths can be C-style quoted.
Unfortunately, the current remote-hg helper doesn't unquote quoted
path and pass them as-is to Mercurial when the commit is created.

This results in the following situation:

 - clone a mercurial repository with git
 - add a file with space in a directory: `>dir/foo\ bar`
 - commit that new file, and push the change to mercurial
 - the mercurial repository now has a new directory named '"dir',
   which contains a file named 'foo bar"'

Use Python str.decode('string-escape') to unquote the string if it
starts and ends with ".  It has been tested with quotes, spaces, and
utf-8 encoded file-names.

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/remote-helpers/git-remote-hg

@@ -678,6 +678,11 @@ def get_merge_files(repo, p1, p2, files):
             f = { 'ctx' : repo[p1][e] }
             files[e] = f
 
+def c_style_unescape(string):
+    if string[0] == string[-1] == '"':
+        return string.decode('string-escape')[1:-1]
+    return string
+
 def parse_commit(parser):
     global marks, blob_marks, parsed_refs
     global mode
@@ -720,6 +725,7 @@ def parse_commit(parser):
             f = { 'deleted' : True }
         else:
             die('Unknown file command: %s' % line)
+        path = c_style_unescape(path).decode('utf-8')
         files[path] = f
 
     # only export the commits if we are on an internal proxy repo