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