commit: remove find_header_mem()

commit: remove find_header_mem()

cfc5cf428b (receive-pack.c: consolidate find header logic, 2022-01-06)
introduced find_header_mem() and turned find_commit_header() into a thin
wrapper.  Since then, the latter has become the last remaining caller of
the former.  Remove it to restore find_commit_header() to the state
before cfc5cf428b, get rid of a strlen(3) call and resolve a NEEDSWORK
note in the process.

Signed-off-by: René Scharfe <l.s.r@web.de>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>