name-rev: don't xstrdup() an already dup'd string

name-rev: don't xstrdup() an already dup'd string

When "add_to_tip_table()" is called with a non-zero
"shorten_unambiguous" we always return an xstrdup()'d string, which
we'd then xstrdup() again, leaking memory. See [1] and [2] for how
this leak came about.

We could xstrdup() only if "shorten_unambiguous" wasn't true, but
let's instead inline this code, so that information on whether we need
to xstrdup() is contained within add_to_tip_table().

1. 98c5c4ad015 (name-rev: allow to specify a subpath for --refs
   option, 2013-06-18)
2. b23e0b9353e (name-rev: allow converting the exact object name at
   the tip of a ref, 2013-07-07)

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