t7610-mergetool: use test_config to isolate tests

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
test-lib-functions: adjust style to match CodingGuidelines

Prefer "test" over "[ ]" for conditionals.
Prefer "$()" over backticks for command substitutions.
Avoid control structures on a single line with semicolons.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>