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

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>