blame.c: fix garbled error message

blame.c: fix garbled error message

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 <redacted>
Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>