reflog tests: add missing "git reflog exists" tests

There were a few "git reflog exists" tests scattered over the test
suite, but let's consolidate the testing of the main functionality
into a new test file. This makes it easier to run just these tests
during development.

To do that amend and extend an existing test added in
afcb2e7a3b (git-reflog: add exists command, 2015-07-21). Let's use
"test_must_fail" instead of "!" (in case it segfaults), and test for
basic usage, an unknown option etc.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1411-reflog-show.sh

@@ -169,9 +169,4 @@ test_expect_success 'git log -g -p shows diffs vs. parents' '
 	test_cmp expect actual
 '
 
-test_expect_success 'reflog exists works' '
-	git reflog exists refs/heads/main &&
-	! git reflog exists refs/heads/nonexistent
-'
-
 test_done

t/t1418-reflog-exists.sh

@@ -0,0 +1,32 @@
+#!/bin/sh
+
+test_description='Test reflog display routines'
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
+export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	test_commit A
+'
+
+test_expect_success 'usage' '
+	test_expect_code 129 git reflog exists &&
+	test_expect_code 129 git reflog exists -h
+'
+
+test_expect_success 'usage: unknown option' '
+	test_expect_code 129 git reflog exists --unknown-option
+'
+
+test_expect_success 'reflog exists works' '
+	git reflog exists refs/heads/main &&
+	test_must_fail git reflog exists refs/heads/nonexistent
+'
+
+test_expect_success 'reflog exists works with a "--" delimiter' '
+	git reflog exists -- refs/heads/main &&
+	test_must_fail git reflog exists -- refs/heads/nonexistent
+'
+
+test_done