merge-ll: consolidate conflict marker scanning logic

merge-ll: consolidate conflict marker scanning logic

The diff.c:is_conflict_marker() and rerere.c:is_cmarker() functions
implement duplicate logic for identifying conflict marker lines
(lines that begin with a run of '<', '=', '>', and '|' characters).

diff.c's original version from 049540435f (diff --check: detect
leftover conflict markers, 2008-06-26) accepts any whitespace (such
as a newline) immediately following '<<<<<<<' and '>>>>>>>', whereas
rerere.c's version from 191f241717 (rerere: prepare for customizable
conflict marker length, 2010-01-16) strictly requires a space
character (' ') after them.

Implement is_conflict_marker_line() in merge-ll.c to serve as a
replacement for both, and update diff.c and rerere.c to use the new
helper.  The unified helper intentionally adopts rerere's stricter
rule, as the conflicts generated by Git always show the "ours" and
"theirs" labels after these markers separated by a space.

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