Merge branch 'da/mergetool-custom'

The actual external command to run for mergetool backend can be
specified with difftool/mergetool.$name.cmd configuration
variables, but this mechanism was ignored for the backends we
natively support.

* da/mergetool-custom:
mergetool--lib: Allow custom commands to override built-ins

diff --cc git-mergetool--lib.sh
Simple merge

diff --cc t/t7610-mergetool.sh
Simple merge