patch: add a new function to initialize diffopts by feature

Not all callers are interested in all diffopts -- for example, commands like
record (which use diff internally) break when diffopts like noprefix are
enabled. This function will allow us to add flags that callers can use to
enable only the features they're interested in.

mercurial/patch.py

@@ -1559,6 +1559,11 b' class GitDiffRequired(Exception):'
     pass
 
 def diffopts(ui, opts=None, untrusted=False, section='diff'):
+    '''return diffopts with all features supported and parsed'''
+    return difffeatureopts(ui, opts=opts, untrusted=untrusted, section=section)
+
+def difffeatureopts(ui, opts=None, untrusted=False, section='diff'):
+    '''return diffopts with only opted-in features parsed'''
     def get(key, name=None, getter=ui.configbool, forceplain=None):
         if opts:
             v = opts.get(key)