The helper functions prepare_final() and prepare_initial() return a
pointer to a string that is a member of an object in the revs->pending
array. This array is later rebuilt when running prepare_revision_walk()
which potentially transforms the pointer target into a bogus string. Fix
this by maintaining a copy of the original string.
Signed-off-by: Lukas Fleischer <git@cryptocrack.de> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>