builtin-rerere: more carefully find conflict markers

builtin-rerere: more carefully find conflict markers

When a conflicting file contains a line that begin with "=======", rerere
failed to parse conflict markers. This result to a wrong preimage file and
an unexpected error for the user.  The boundary between ours and theirs
not just begin with 7 equals, but is followed by either a SP or a LF.

This patch enforces parsing rules so that markers match in the right order,
and when ambiguous, the command does not autoresolve the conflicted file.

Especially because we are introducing rerere.autoupdate configuration
(which is off by default for safety) that automatically stages the
resolution made by rerere, it is necessary to make sure that we do not
autoresolve when there is any ambiguity.

Signed-off-by: Olivier Marin <dkr@freesurf.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>