Turns out we always need to set the ignored prefix (compset) to have
similar behavior as in default Bash.
The issue can be seen with:
git show master:<tab>
Commit 94b2901cfe wrongly removed it.
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>