ui: add diff.showfunc to tweakdefaults

This is a little risky, as I think we can have some encoding weirdness
crop up. showfunc also isn't the most robust feature, but it's still
often useful context...

Differential Revision: https://phab.mercurial-scm.org/D1610

mercurial/ui.py

@@ -62,6 +62,7 @@ update.check = noconflict
 
 [diff]
 git = 1
+showfunc = 1
 """
 
 samplehgrcs = {