Drop the requirement of P4DIFF="diff -u" for perforce workspace

"p4 diff -du" does the job. Also switch to "p4 info" which is slightly
faster.

README.rst

@@ -192,9 +192,7 @@ Redirect output to another patch file is safe even without ``-u``:
 Notes
 -----
 
-1. Ydiff only supports diffs in `Unified Format`_. This is default in most
-   version control system except Perforce, which needs an environment variable
-   ``P4DIFF="diff -u"`` to output unified diff.
+1. Ydiff only supports diffs in `Unified Format`_.
 
    .. _`Unified Format`: https://en.wikipedia.org/wiki/Diff#Unified_format
 

ydiff.py

@@ -671,8 +671,8 @@ def markup_to_pager(stream, opts):
         'log': ['hg', 'log', '--patch'],
     },
     'Perforce': {
-        'probe': ['p4', 'dirs', '.'],
-        'diff': ['p4', 'diff'],
+        'probe': ['p4', 'info'],
+        'diff': ['p4', 'diff', '-du'],
         'log': None,
     },
     'Svn': {