t1415: avoid using `main` as ref name

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: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>