builtin/merge: always store allocated strings in `pull_twohead`

builtin/merge: always store allocated strings in `pull_twohead`

The `pull_twohead` configuration may sometimes contain an allocated
string, and sometimes it may contain a string constant. Refactor this to
instead always store an allocated string such that we can release its
resources without risk.

While at it, manage the lifetime of other config strings, as well. Note
that we explicitly don't free `cleanup_arg` here. This is because the
variable may be assigned a string constant via command line options.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>