worktree: fix a trivial leak in prune_worktrees()

worktree: fix a trivial leak in prune_worktrees()

We were leaking both the "struct strbuf" in prune_worktrees(), as well
as the "path" we got from should_prune_worktree(). Since these were
the only two uses of the "struct string_list" let's change it to a
"DUP" and push these to it with "string_list_append_nodup()".

For the string_list_append_nodup() we could also string_list_append()
the main_path.buf, and then strbuf_release(&main_path) right away. But
doing it this way avoids an allocation, as we already have the "struct
strbuf" prepared for appending to "kept".

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