am: remove rebase-apply directory before gc

When git am does an automatic gc it doesn't clean up the rebase-apply
directory until after this has finished.  This means that if the user
aborts the gc then future am or rebase operations will report that an
existing operation is in progress, which is undesirable and confusing.

Reported by Mark Brown <broonie@debian.org> through
http://bugs.debian.org/570966

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -776,6 +776,5 @@ do
 	go_next
 done
 
-git gc --auto
-
 rm -fr "$dotest"
+git gc --auto