merge: optimization to skip evaluate_result for single strategy

For a merge with a single strategy, the result of evaluate_result() is
effectively not used and therefore is not needed, so avoid altogether.

On Windows, this optimization can halve the time required to perform a
recursive merge of a single commit with the LLVM repo.

Signed-off-by: Andrew Ng <andrew.ng@sony.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -1629,7 +1629,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 				}
 				merge_was_ok = 1;
 			}
-			cnt = evaluate_result();
+			cnt = (use_strategies_nr > 1) ? evaluate_result() : 0;
 			if (best_cnt <= 0 || cnt <= best_cnt) {
 				best_strategy = use_strategies[i]->name;
 				best_cnt = cnt;