builtin/fast-export: plug leaking tag names

builtin/fast-export: plug leaking tag names

When resolving revisions in `get_tags_and_duplicates()`, we only
partially manage the lifetime of `full_name`. In fact, managing its
lifetime properly is almost impossible because we put direct pointers to
that variable into multiple lists without duplicating the string. The
consequence is that these strings will ultimately leak.

Refactor the code to make the lists we put those names into duplicate
the memory. This allows us to properly free the string as required and
thus plugs the memory leak.

While this requires us to allocate more data overall, it shouldn't be
all that bad given that the number of allocations corresponds with the
number of command line parameters, which typically aren't all that many.

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