git-am: use printf instead of echo on user-supplied strings

Under some implementations of echo (such as that provided by
dash), backslash escapes are recognized without any other
options. This means that echo-ing user-supplied strings may
cause any backslash sequences in them to be converted. Using
printf resolves the ambiguity.

This bug can be seen when using git-am to apply a patch
whose subject contains the character sequence "\n"; the
characters are converted to a literal newline. Noticed by
Szekeres Istvan.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-am.sh

@@ -331,7 +331,7 @@ do
 		ADD_SIGNOFF=
 	    fi
 	    {
-		echo "$SUBJECT"
+		printf '%s\n' "$SUBJECT"
 		if test -s "$dotest/msg-clean"
 		then
 			echo
@@ -394,7 +394,7 @@ do
 	fi
 
 	echo
-	echo "Applying '$SUBJECT'"
+	printf 'Applying %s\n' "$SUBJECT"
 	echo
 
 	case "$resolved" in