git-am: head -1 is obsolete and doesn't work on some new systems

head -<n> was deprecated by POSIX, and as modern versions of coreutils
package don't support it at least one exports _POSIX2_VERSION=199209
it's fails on some systems.

head -n<n> is portable, but sed <n>q is even more.

Signed-off-by: Alejandro Mery <amery@geeks.cl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -421,7 +421,7 @@ do
 	else
 	    action=yes
 	fi
-	FIRSTLINE=$(head -1 "$dotest/final-commit")
+	FIRSTLINE=$(sed 1q "$dotest/final-commit")
 
 	if test $action = skip
 	then