grep: stop computing information for --diff when unnecessary

This is one reason why `hg grep pattern` essentially does `hg cat -r
. 'set:**'` inside. There is no speed improvement in this commit,
because the rest of the code still greps data from filelog instead of
working copy when possible.

Differential Revision: https://phab.mercurial-scm.org/D8544

mercurial/commands.py

@@ -3594,8 +3594,9 b' def grep(ui, repo, pattern, *pats, **opt'
     def prep(ctx, fns):
         rev = ctx.rev()
         pctx = ctx.p1()
+        matches.setdefault(rev, {})
+        if diff:
         parent = pctx.rev()
-        matches.setdefault(rev, {})
         matches.setdefault(parent, {})
         files = revfiles.setdefault(rev, [])
         for fn in fns:
@@ -3620,11 +3621,14 b' def grep(ui, repo, pattern, *pats, **opt'
                 content = get_file_content(fn, flog, fnode, ctx, rev)
                 grepbody(fn, rev, content)
 
+            if diff:
             pfn = copy or fn
             if pfn not in matches[parent]:
                 try:
                     pfnode = pctx.filenode(pfn)
-                    pcontent = get_file_content(pfn, flog, pfnode, pctx, parent)
+                        pcontent = get_file_content(
+                            pfn, flog, pfnode, pctx, parent
+                        )
                     grepbody(pfn, parent, pcontent)
                 except error.LookupError:
                     pass