[GSoC][RFC WIP PATCH 3/3] grep: re-enable threads in non-worktree case

[Matheus Tavares <matheus.bernardino@usp.br>]

They were disabled at 53b8d93 ("grep: disable threading in non-worktree
case", 12-12-2011), due to observable perfomance drops. But now that
zlib inflation can be performed in parallel, we can regain the speedup.

Grep'ing 'abcd[02]' ("Regex 1") and '(static|extern) (int|double) \*'
("Regex 2") at chromium's repository[1] I got:

 Threads |   Regex 1  |  Regex 2
---------|------------|-----------
    1    |  17.5815s  | 21.7217s
    2    |   9.7983s  | 11.3965s
    8    |   6.3097s  |  6.9667s

These are all means of 30 executions after 2 warmup runs. All tests were
executed on a i7-7700HQ with 16GB of RAM and SSD.

[1]: chromium’s repo at commit 03ae96f (“Add filters testing at DSF=2”,
     04-06-2019), after a 'git gc' execution.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
---
 builtin/grep.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/builtin/grep.c b/builtin/grep.c
index 682e2461d0..9309dea833 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -1046,7 +1046,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	pathspec.recursive = 1;
 	pathspec.recurse_submodules = !!recurse_submodules;
 
-	if (list.nr || cached || show_in_pager) {
+	if (show_in_pager) {
 		if (num_threads > 1)
 			warning(_("invalid option combination, ignoring --threads"));
 		num_threads = 1;
-- 
2.22.0