eol: improve hook failure output
Before, only the first failure was reported:
abort: b.txt should not have CRLF line endings
while now all of them are listed:
abort: end-of-line check failed:
d.txt in a7040e68714f should not have CRLF line endings
b.txt in fbcf9b1025f5 should not have CRLF line endings
As first suggested by Antoine Pitrou <solipsis@pitrou.net>