Merge branch 'rt/help-unknown'

"git nosuchcommand --help" said "No manual entry for gitnosuchcommand",
which was not intuitive, given that "git nosuchcommand" said "git:
'nosuchcommand' is not a git command".

* rt/help-unknown:
  help: make option --help open man pages only for Git commands
  help: introduce option --exclude-guides

diff --cc git.c
Simple merge