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>

Documentation/git-log.txt

@@ -37,7 +37,8 @@ include::diff-options.txt[]
 	and <until>, see "SPECIFYING REVISIONS" section in
 	linkgit:git-rev-parse[1].
 
---decorate[=short|full]::
+--no-decorate::
+--decorate[=short|full|no]::
 	Print out the ref names of any commits that are shown. If 'short' is
 	specified, the ref name prefixes 'refs/heads/', 'refs/tags/' and
 	'refs/remotes/' will not be printed. If 'full' is specified, the