am: Use cat instead of echo to avoid DOS line-endings (fixes t4150)

Along the lines of 05d0e3b and f33946d, use cat instead of echo to avoid
line ending mismatches in the test result of "am empty-file does not
infloop" which make the test fail.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>

git-am.sh

@@ -184,7 +184,9 @@ It does not apply to blobs recorded in its index.")"
 }
 
 clean_abort () {
-	test $# = 0 || echo >&2 "$@"
+	test $# = 0 || cat >&2 <<EOF
+$@
+EOF
 	rm -fr "$dotest"
 	exit 1
 }