t7607: use test-lib functions and check MERGE_HEAD

t7607: use test-lib functions and check MERGE_HEAD

Use the test_commit and test_path_is_missing
functions from the test library.

Also make sure that a merge which fails due to
pre-merge checks aborts properly and does not
leave MERGE_HEAD behind.

The "will not overwrite removed file" test is an
exception to this. It notices the untracked file
at a stage where the merge is already well under
way. Therefore we cannot abort the merge without
major restructuring. See the following thread for
more details.

http://mid.gmane.org/7vskopwxej.fsf@gitster.siamese.dyndns.org

Signed-off-by: Clemens Buchacher <drizzd@aon.at>