t7600 (merge): do not launch gitk for --debug

t7600 (merge): do not launch gitk for --debug

Probably as a development aid, this test script runs gitk --all
to allow the driver to inspect history between tests when run
with --debug.  As a result, running all tests with --debug
requires closing a long series of gitk displays, one at a time.

Use git log --graph --oneline instead.  This way, the history is
available for viewing with "git show" but the test script finishes
without interaction.

Longer term, it would be nice to have an option to run a
user-specified command between tests.  This patch does not do
that.

Cc: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>