template: call runtemplate on the src argument to sub

This allows using a template keyword on calls to the sub function and brings
the function inline with most other semantics of the other template functions.

mercurial/templater.py

@@ -251,6 +251,8 b' def sub(context, mapping, args):'
     pat = stringify(args[0][0](context, mapping, args[0][1]))
     rpl = stringify(args[1][0](context, mapping, args[1][1]))
     src = stringify(args[2][0](context, mapping, args[2][1]))
+    src = stringify(runtemplate(context, mapping,
+                                compiletemplate(src, context)))
     yield re.sub(pat, rpl, src)
 
 def if_(context, mapping, args):

tests/test-command-template.t

@@ -1531,3 +1531,7 b' Test new-style inline templating:'
   $ hg log -R latesttag -r tip --template 'modified files: {file_mods % " {file}\n"}\n'
   modified files:  .hgtags
   
+Test the sub function of templating for expansion:
+
+  $ hg log -R latesttag -r 10 --template '{sub("[0-9]", "x", "{rev}")}\n'
+  xx