diff: teach diff to read algorithm from diff driver

It can be useful to specify diff algorithms per file type. For example,
one may want to use the minimal diff algorithm for .json files, another
for .c files, etc.

The diff machinery already checks attributes for a diff driver. Teach
the diff driver parser a new type "algorithm" to look for in the
config, which will be used if a driver has been specified through the
attributes.

Enforce precedence of the diff algorithm by favoring the command line
option, then looking at the driver attributes & config combination, then
finally the diff.algorithm config.

To enforce precedence order, use a new `ignore_driver_algorithm` member
during options parsing to indicate the diff algorithm was set via command
line args.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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