The clean-up of this test script was long overdue and is a very
welcome change.
* da/mergetool-tests:
test-lib-functions: adjust style to match CodingGuidelines
t7610-mergetool: use test_config to isolate tests
t7610-mergetool: add missing && and remove commented-out code
t7610-mergetool: use tabs instead of a mix of tabs and spaces