t3903: add test for --intent-to-add file

t3903: add test for --intent-to-add file

Add a test showing the 'git stash' behaviour with a file that has been
added with 'git add --intent-to-add'.  Stash fails to stash the file,
so the purpose of this test is mainly to make sure git doesn't crash,
but exits normally in this situation.

This is in preparation for converting stash into a builtin.

[tg: pulled the test out into a separate commit]

Signed-off-by: Matthew Kraai <mkraai@its.jnj.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>