rm: better error message on failure for multiple files

rm: better error message on failure for multiple files

When 'git rm' fails, it now displays a single message
with the list of files involved, instead of displaying
a list of messages with one file each.

As an example, the old message:
	error: 'foo.txt' has changes staged in the index
	(use --cached to keep the file, or -f to force removal)
	error: 'bar.txt' has changes staged in the index
	(use --cached to keep the file, or -f to force removal)

would now be displayed as:
	error: the following files have changes staged in the index:
	    foo.txt
	    bar.txt
	(use --cached to keep the file, or -f to force removal)

Signed-off-by: Mathieu Lienard--Mayor <Mathieu.Lienard--Mayor@ensimag.imag.fr>
Signed-off-by: Jorge Juan Garcia Garcia <Jorge-Juan.Garcia-Garcia@ensimag.imag.fr>
Signed-off-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>