git-am: head -1 is obsolete and doesn't work on some new systems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

---
 git-am.sh |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

index b48096e..2c517ed 100755
@@ -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

--
1.5.4.3