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

diff --combined contrib/completion/git-completion.bash
index 5f556075d3cbce74c60e81b97d4923a1ff18f78d,a45b4a0508b7001cefbf82680068dc231a57dbe0..86b8f474c8d5af0e5a948b835ba104fd963cbff5
--- 1/contrib/completion/git-completion.bash
--- 2/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@@ -1341,6 -1341,17 +1341,6 @@@ _git_checkout (
        esac
  }
  
 -_git_cherry ()
 -{
 -      case "$cur" in
 -      --*)
 -              __gitcomp_builtin cherry
 -              return
 -      esac
 -
 -      __git_complete_refs
 -}
 -
  __git_cherry_pick_inprogress_options="--continue --quit --abort"
  
  _git_cherry_pick ()
@@@ -1822,7 -1833,7 +1822,7 @@@ _git_mergetool (
                return
                ;;
        --*)
-               __gitcomp "--tool= --prompt --no-prompt"
+               __gitcomp "--tool= --prompt --no-prompt --gui --no-gui"
                return
                ;;
        esac