Merge branch 'mj/log-show-signature-conf'

"git log" learns log.showSignature configuration variable, and a
command line option "--no-show-signature" to countermand it.

* mj/log-show-signature-conf:
  log: add log.showSignature configuration variable
  log: add "--no-show-signature" command line option
  t4202: refactor test

Documentation/git-log.txt

@@ -198,6 +198,10 @@ log.showRoot::
 	`git log -p` output would be shown without a diff attached.
 	The default is `true`.
 
+log.showSignature::
+	If `true`, `git log` and related commands will act as if the
+	`--show-signature` option was passed to them.
+
 mailmap.*::
 	See linkgit:git-shortlog[1].