log.decorate: usability fixes

log.decorate: usability fixes

The configuration is meant to suppliment --decorate command line option
that can be used as a boolean to turn the feature on, so it is natural
to expect

	[log]
		decorate
		decorate = yes

to work.  The original commit would segfault with the first one, and
would not understand the second one.

Once a user has this configuration in ~/.gitconfig, there needs to be a
way to override it from the command line.  Add --no-decorate option to
log family and also allow --decorate=no to mean the same thing.  Since
we allow setting log.decorate to 'true', the command line also should
accept --decorate=yes and behave accordingly.

New tests in t4202 are designed to exercise the interaction between the
configuration variable and the command line option that overrides it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>