Merge branch 'dl/mergetool-gui-option'

"git mergetool" learned to take the "--[no-]gui" option, just like
"git difftool" does.

* dl/mergetool-gui-option:
doc: document diff/merge.guitool config keys
completion: support `git mergetool --[no-]gui`
mergetool: accept -g/--[no-]gui as arguments