stash: always have the owner of "stash_info" free it

stash: always have the owner of "stash_info" free it

Change the initialization of the "revision" member of "struct
stash_info" to be initialized vi a macro, and more importantly that
that initializing function be tasked to free it, usually via a "goto
cleanup" pattern.

Despite the "revision" name (and the topic of the series containing
this commit) the "stash info" has nothing to do with the "struct
rev_info". I'm making this change because in the subsequent commit
when we do want to free the "struct rev_info" via a "goto cleanup"
pattern we'd otherwise free() uninitialized memory in some cases, as
we only strbuf_init() the string in get_stash_info().

So while it's not the smallest possible change, let's convert all
users of this pattern in the file while we're at it.

A good follow-up to this change would be to change all the "ret = -1;
goto done;" in this file to instead use a "goto cleanup", and
initialize "int ret = -1" at the start of the relevant functions. That
would allow us to drop a lot of needless brace verbosity on two-line
"if" statements, but let's leave that alone for now.

To ensure that there's a 1=1 mapping between owners of the "struct
stash_info" and free_stash_info() change the assert_stash_ref()
function to be a trivial get_stash_info_assert() wrapper. The caller
will call free_stash_info(), and by returning -1 we'll eventually (via
!!ret) exit with status 1 anyway.

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