t6026-merge-attr: clean up background process at end of test case

t6026-merge-attr: clean up background process at end of test case

The process spawned in the hook uses the test's trash directory as CWD.
As long as it is alive, the directory cannot be removed on Windows.
Although the test succeeds, the 'test_done' that follows produces an
error message and leaves the trash directory around. Kill the process
before the test case advances.

Helped-by: Johannes Schindelin <redacted>
Signed-off-by: Johannes Sixt <redacted>
Signed-off-by: Junio C Hamano <redacted>