builtin/log: honor log.decorate

builtin/log: honor log.decorate

The recent change that introduced autodecorating of refs accidentally
broke the ability of users to set log.decorate = false to override it.
When the git_log_config was traversed a second time with an option other
than log.decorate, the decoration style would be set to the automatic
style, even if the user had already overridden it.  Instead of setting
the option in config parsing, set it in init_log_defaults instead.

Add a test for this case.  The actual additional config option doesn't
matter, but it needs to be something not already set in the
configuration file.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Acked-by: Alex Henrie <alexhenrie24@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>