stash: add coverage for show --include-untracked

stash: add coverage for show --include-untracked

Add a test for 'git stash show --include-untracked' to
cover the case where untracked files saved in the stash
are included in the output.

While stash creation and restoration of untracked files
are already tested, there is currently no explicit test
covering the output behavior of 'stash show
--include-untracked'.

Signed-off-by: Pushkar Singh <pushkarkumarsingh1970@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>