checkout: avoid "struct unpack_trees_options" leak

checkout: avoid "struct unpack_trees_options" leak

In 1c41d2805e4 (unpack_trees_options: free messages when done,
2018-05-21) we started calling clear_unpack_trees_porcelain() on this
codepath, but missed this error path.

We could call clear_unpack_trees_porcelain() just before we error()
and return when unmerged_cache() fails, but the more correct fix is to
not have the unmerged_cache() check happen in the middle of our
"topts" setup.

Before 23cbf11b5c0 (merge-recursive: porcelain messages for checkout,
2010-08-11) we would not malloc() to setup our "topts", which is when
this started to leak on the error path.

Before that this code wasn't conflating the setup of "topts" and the
unmerged_cache() call in any meaningful way. The initial version in
782c2d65c24 (Build in checkout, 2008-02-07) just does a "memset" of
it, and initializes a single struct member.

Then in 8ccba008ee3 (unpack-trees: allow Porcelain to give different
error messages, 2008-05-17) we added the initialization of the error
message, which as noted above finally started leaking in 23cbf11b5c0.

Let's fix the memory leak, and avoid future issues by initializing the
"topts" with a helper function. There are no functional changes here.

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