Merge branch 'da/mergetool-tests'

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

Author: gitster