grep: exit loop immediately, if matching is found in the file for "hg grep -l"

Before this patch, internal function "display()" of "hg grep" is not
efficient for "-l"/"--files-with-matches", because loop is continued,
even after the first matching is found in the specified file.

This patch exits loop immediately, if matching is found for
"--files-with-matches".

In this case, "before is None" is equal to "opts.get('files_with_matches')".

mercurial/commands.py

@@ -3364,10 +3364,7 b' def grep(ui, repo, pattern, *pats, **opt'
                 cols.append((ui.shortuser(ctx.user()), 'grep.user'))
             if opts.get('date'):
                 cols.append((datefunc(ctx.date()), 'grep.date'))
-            if opts.get('files_with_matches'):
-                if found:
-                    continue
-            else:
+            if not opts.get('files_with_matches'):
                 before = l.line[:l.colstart]
                 match = l.line[l.colstart:l.colend]
                 after = l.line[l.colend:]
@@ -3385,6 +3382,8 b' def grep(ui, repo, pattern, *pats, **opt'
                     ui.write(after)
             ui.write(eol)
             found = True
+            if before is None:
+                break
         return found
 
     skip = {}