t1415: avoid using `main` as ref name

In preparation for a patch series that will change the fall-back for
`init.defaultBranch` to `main`, let's not use `main` as ref name in this
test script.

Otherwise, the `git for-each-ref ... | grep main` which wants to catch
those refs would also unexpectedly catch `refs/heads/main`.

Since the refs in question are worktree-local ones (i.e. each worktree
has their own, just like `HEAD`), and since the test case already uses a
secondary worktree called "second", let's use the name "first" for those
refs instead.

While at it, adjust the test titles that talk about a "repo" when they
meant a "worktree" instead.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t1415-worktree-refs.sh

@@ -76,24 +76,24 @@ test_expect_success 'reflog of worktrees/xx/HEAD' '
 	test_cmp expected actual.wt2
 '
 
-test_expect_success 'for-each-ref from main repo' '
+test_expect_success 'for-each-ref from main worktree' '
 	mkdir fer1 &&
 	git -C fer1 init repo &&
 	test_commit -C fer1/repo initial &&
 	git -C fer1/repo worktree add ../second &&
-	git -C fer1/repo update-ref refs/bisect/main HEAD &&
-	git -C fer1/repo update-ref refs/rewritten/main HEAD &&
-	git -C fer1/repo update-ref refs/worktree/main HEAD &&
-	git -C fer1/repo for-each-ref --format="%(refname)" | grep main >actual &&
+	git -C fer1/repo update-ref refs/bisect/first HEAD &&
+	git -C fer1/repo update-ref refs/rewritten/first HEAD &&
+	git -C fer1/repo update-ref refs/worktree/first HEAD &&
+	git -C fer1/repo for-each-ref --format="%(refname)" | grep first >actual &&
 	cat >expected <<-\EOF &&
-	refs/bisect/main
-	refs/rewritten/main
-	refs/worktree/main
+	refs/bisect/first
+	refs/rewritten/first
+	refs/worktree/first
 	EOF
 	test_cmp expected actual
 '
 
-test_expect_success 'for-each-ref from linked repo' '
+test_expect_success 'for-each-ref from linked worktree' '
 	mkdir fer2 &&
 	git -C fer2 init repo &&
 	test_commit -C fer2/repo initial &&