log/show/whatchanged: introduce format.pretty configuration

log/show/whatchanged: introduce format.pretty configuration

When running log/show/whatchanged from the command line, the user may
want to use a preferred format without having to pass --pretty=<fmt>
option every time from the command line.  This teaches these three
commands to honor a new configuration variable, format.pretty.

The --pretty option given from the command line will override the
configured format.

The earlier patch fixed the in-tree callers that run these commands
for purposes other than showing the output directly to the end user
(the only other in-tree caller is "git bisect visualize", whose output
directly goes to the end user and should be affected by this patch).

Similar fixes will be needed for end-user scripts that parse the
output from these commands and expect them to be in the default pretty
format.

Signed-off-by: Denis Cheng <crquan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>