mdiff.diffopts: add a new noprefix option

By popular demand, we introduce an option to disable the 'a/' and 'b/'
prefixes in diff output. This makes copying and pasting filenames from diff
output easier.

This option will be implemented and documented in upcoming patches. To ensure
that existing scripts that parse output don't break, we will ensure that this
prefix is disabled in plain mode. A straight 'hg export | hg import' without
HGPLAIN=1 will still be broken though, but there's little that can be done
about that.

mercurial/mdiff.py

@@ -26,6 +26,7 b' class diffopts(object):'
     git enables the git extended patch format
     nodates removes dates from diff headers
     nobinary ignores binary files
+    noprefix disables the 'a/' and 'b/' prefixes (ignored in plain mode)
     ignorews ignores all whitespace changes in the diff
     ignorewsamount ignores changes in the amount of whitespace
     ignoreblanklines ignores changes whose lines are all blank
@@ -39,6 +40,7 b' class diffopts(object):'
         'git': False,
         'nodates': False,
         'nobinary': False,
+        'noprefix': False,
         'ignorews': False,
         'ignorewsamount': False,
         'ignoreblanklines': False,