If the libexec directory doesn't exist, git-subtree gets installed as
$prefix/share/libexec/git-core file. This patch creates the directory
before installing git-subtree file into it.
Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz> Signed-off-by: Junio C Hamano <gitster@pobox.com>