diff -r cecbe207cebd -r 554d6fcc3c84 mercurial/templater.py
--- a/mercurial/templater.py	Thu May 07 23:18:48 2015 -0700
+++ b/mercurial/templater.py	Fri May 08 18:11:26 2015 +0900
@@ -623,7 +623,21 @@
     if quoted:
         if len(s) < 2 or s[0] != s[-1]:
             raise SyntaxError(_('unmatched quotes'))
-        return s[1:-1]
+        # de-backslash-ify only <\">. it is invalid syntax in non-string part of
+        # template, but we are likely to escape <"> in quoted string and it was
+        # accepted before, thanks to issue4290. <\\"> is unmodified because it
+        # is ambiguous and it was processed as such before 2.8.1.
+        #
+        #  template  result
+        #  --------- ------------------------
+        #  {\"\"}    parse error
+        #  "{""}"    {""} -> <>
+        #  "{\"\"}"  {""} -> <>
+        #  {"\""}    {"\""} -> <">
+        #  '{"\""}'  {"\""} -> <">
+        #  "{"\""}"  parse error (don't care)
+        q = s[0]
+        return s[1:-1].replace('\\\\' + q, '\\\\\\' + q).replace('\\' + q, q)
 
     return s