git-bisect: war on "sed"

git-bisect: war on "sed"

We do not need to pipe "echo" to "sed" only to strip refs/heads/
from the beginning.  We are assuming not-so-ancient shells these
days.

Also there is no need to avoid assuming \012 is the LF; we do
not run on EBCDIC, sorry.  Other parts of the script already
uses tr to convert separator to LF that way.

Signed-off-by: Junio C Hamano <gitster@pobox.com>