grep: don't look up copy info unless --follow is given

If no --follow was given, then the "copy" variable will become
None. In that case we would still look up the copy information from
the filelog and then ignore it. Let's avoid even looking it up.

Differential Revision: https://phab.mercurial-scm.org/D5620

mercurial/commands.py

@@ -2944,16 +2944,18 b' def grep(ui, repo, pattern, *pats, **opt'
                 fnode = ctx.filenode(fn)
             except error.LookupError:
                 continue
-            try:
-                copied = flog.renamed(fnode)
-            except error.WdirUnsupported:
-                copied = ctx[fn].renamed()
-            copy = follow and copied and copied[0]
-            if copy:
-                copies.setdefault(rev, {})[fn] = copy
+            copy = None
+            if follow:
+                try:
+                    copied = flog.renamed(fnode)
+                except error.WdirUnsupported:
+                    copied = ctx[fn].renamed()
+                copy = copied and copied[0]
+                if copy:
+                    copies.setdefault(rev, {})[fn] = copy
+                    if fn in skip:
+                        skip[copy] = True
             if fn in skip:
-                if copy:
-                    skip[copy] = True
                 continue
             files.append(fn)