Custom low-level merge driver: change the configuration scheme.

Custom low-level merge driver: change the configuration scheme.

This changes the configuration syntax for defining a low-level
merge driver to be:

	[merge "<<drivername>>"]
		driver = "<<command line>>"
		name = "<<driver description>>"

which is much nicer to read and is extensible.  Credit goes to
Martin Waitz and Linus.

In addition, when we use an external low-level merge driver, it
is reported as an extra output from merge-recursive, using the
value of merge.<<drivername>.name variable.

The demonstration in t6026 has also been updated.

Signed-off-by: Junio C Hamano <junkio@cox.net>