diff: when looking for diff configs, support a configurable prefix

In a future commit, I want to make it possible to have the diff options pulled
from (as an example) `commands.commit.interactive.ignorews`; previously we only
supported this for customizable sections (so this would have needed a
`commit-interactive` section and been named `commit-interactive.ignorews`, which
felt a bit weird.

Differential Revision: https://phab.mercurial-scm.org/D5833

mercurial/diffutil.py

@@ -16,13 +16,15 b' from . import ('
     pycompat,
 )
 
-def diffallopts(ui, opts=None, untrusted=False, section='diff'):
+def diffallopts(ui, opts=None, untrusted=False, section='diff',
+                configprefix=''):
     '''return diffopts with all features supported and parsed'''
     return difffeatureopts(ui, opts=opts, untrusted=untrusted, section=section,
-                           git=True, whitespace=True, formatchanging=True)
+                           git=True, whitespace=True, formatchanging=True,
+                           configprefix=configprefix)
 
 def difffeatureopts(ui, opts=None, untrusted=False, section='diff', git=False,
-                    whitespace=False, formatchanging=False):
+                    whitespace=False, formatchanging=False, configprefix=''):
     '''return diffopts with only opted-in features parsed
 
     Features:
@@ -45,7 +47,8 b' def difffeatureopts(ui, opts=None, untru'
                 return v
         if forceplain is not None and ui.plain():
             return forceplain
-        return getter(section, name or key, untrusted=untrusted)
+        return getter(section, configprefix + (name or key),
+                      untrusted=untrusted)
 
     # core options, expected to be understood by every diff parser
     buildopts = {