revision: make get_revision_mark() return const pointer

revision: make get_revision_mark() return const pointer

get_revision_mark() used to return a `char *`, even though all of the
strings it was returning were string literals. Make get_revision_mark()
return a `const char *` so that callers won't be tempted to modify the
returned string.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>