Merge branch 'dl/difftool-mergetool'

Update "git difftool" and "git mergetool" so that the combinations of {diff,merge}.{tool,guitool} configuration variables serve as fallback settings of each other in a sensible order. * dl/difftool-mergetool: difftool: fallback on merge.guitool difftool: make --gui, --tool and --extcmd mutually exclusive mergetool: fallback to tool when guitool unavailable mergetool--lib: create gui_mode function mergetool: use get_merge_tool function t7610: add mergetool --gui tests t7610: unsuppress output