t4151: am --abort will keep dirty index intact

t4151: am --abort will keep dirty index intact

Since 7b3b7e3 (am --abort: keep unrelated commits since the last failure
and warn, 2010-12-21), git-am --abort will not touch the index if on the
previous invocation, git-am failed because the index is dirty. This is
to ensure that the user's modifications to the index are not discarded.

Add a test for this.

Reviewed-by: Stefan Beller <redacted>
Reviewed-by: Johannes Schindelin <redacted>
Signed-off-by: Paul Tan <redacted>
Signed-off-by: Junio C Hamano <redacted>