diffs: use custom lexer extraction in diffs to so it behaves consistently with

file source view.

rhodecode/lib/codeblocks.py

@@ -25,7 +25,7 b' from itertools import groupby'
 from pygments import lex
 from pygments.formatters.html import _get_ttype_class as pygment_token_class
 from rhodecode.lib.helpers import (
-    get_lexer_for_filenode, html_escape)
+    get_lexer_for_filenode, html_escape, get_custom_lexer)
 from rhodecode.lib.utils2 import AttributeDict
 from rhodecode.lib.vcs.nodes import FileNode
 from rhodecode.lib.diff_match_patch import diff_match_patch
@@ -407,8 +407,12 b' class DiffSet(object):'
         if filename not in self._lexer_cache:
             if filenode:
                 lexer = filenode.lexer
+                extension = filenode.extension
             else:
                 lexer = FileNode.get_lexer(filename=filename)
+                extension = filename.split('.')[-1]
+
+            lexer = get_custom_lexer(extension) or lexer
             self._lexer_cache[filename] = lexer
         return self._lexer_cache[filename]