include/git: fix error caused by sh idiosyncrasy

when [] is used right at the end of a function, or
certain loops/subshells, some sh implementations will
just return a non-zero exit

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/git.sh

@@ -141,6 +141,7 @@ git_am_patches()
 	)
 	for patches in "${patchdir}/"*; do
 		[ -L "${patches}" ] && continue
-		[ -d "${patches}" ] && git_am_patches "${sdir}" "${patches}"
+		[ ! -d "${patches}" ] && continue
+		git_am_patches "${sdir}" "${patches}"
 	done
 }