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

diff --cc Documentation/git-log.txt
Simple merge

diff --cc builtin/log.c
Simple merge

diff --cc revision.c
Simple merge