Use extended SHA1 syntax in merge-recursive conflicts.

Use extended SHA1 syntax in merge-recursive conflicts.

When we get a line-level conflict in merge-recursive and print out
the two sides in the conflict hunk header and footer we should use
the standard extended SHA1 syntax to specify the specific blob,
as this allows the user to copy and paste the line right into
'git show' to view the complete version.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>