grep: reject --diff --all-files

This combination doesn't make any sense since --diff is the flag to search
change history, whereas --all-files is to include unmodified contents.

Differential Revision: https://phab.mercurial-scm.org/D3916

mercurial/commands.py

@@ -2533,6 +2533,9 b' def grep(ui, repo, pattern, *pats, **opt'
     """
     opts = pycompat.byteskwargs(opts)
     diff = opts.get('all') or opts.get('diff')
+    if diff and opts.get('all_files'):
+        raise error.Abort(_('--diff and --all-files are mutually exclusive'))
+
     reflags = re.M
     if opts.get('ignore_case'):
         reflags |= re.I

tests/test-grep.t

@@ -412,6 +412,12 b' Test for showing working of allfiles fla'
   $ hg grep -r "." "unmod" --all-files
   um:1:unmod
 
+--diff --all-files makes no sense since --diff is the option to grep history
+
+  $ hg grep --diff --all-files um
+  abort: --diff and --all-files are mutually exclusive
+  [255]
+
   $ cd ..
 
 Fix_Wdir(): test that passing wdir() t -r flag does greps on the