A subsequent commit will need the ability to tell if the diff algorithm
was set through the command line through setting a new member of
diff_options. While this logic can be added to the
diff_opt_diff_algorithm() callback, the `--minimal` and `--histogram`
options are handled via OPT_BIT without a callback.

Remedy this by consolidating the options parsing logic for --minimal and
--histogram into one callback. This way we can modify `diff_options` in
that function.

As an additional refactor, the logic that sets the diff algorithm in
diff_opt_diff_algorithm() can be refactored into a helper that will
allow multiple callsites to set the diff algorithm.

Signed-off-by: John Cai <johncai86@gmail.com>

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>