Merge branch 'ac/log-use-mailmap-by-default-transition'

The "git log" command learns to issue a warning when log.mailmap
configuration is not set and --[no-]mailmap option is not used, to
prepare users for future versions of Git that uses the mailmap by
default.

* ac/log-use-mailmap-by-default-transition:
  tests: defang pager tests by explicitly disabling the log.mailmap warning
  documentation: mention --no-use-mailmap and log.mailmap false setting
  log: add warning for unspecified log.mailmap setting