hooks: convert worktree 'post-checkout' hook to hook library

Move the running of the 'post-checkout' hook away from run-command.h
to the new hook.h library in builtin/worktree.c. For this special case
we need a change to the hook API to teach it to run the hook from a
given directory.

We cannot skip the "absolute_path" flag and just check if "dir" is
specified as we'd then fail to find our hook in the new dir we'd
chdir() to. We currently don't have a use-case for running a hook not
in our "base" repository at a given absolute path, so let's have "dir"
imply absolute_path(find_hook(hook_name)).

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

hook.h

@@ -12,6 +12,12 @@ struct run_hooks_opt
 
 	/* Emit an error if the hook is missing */
 	unsigned int error_if_missing:1;
+
+	/**
+	 * An optional initial working directory for the hook,
+	 * translates to "struct child_process"'s "dir" member.
+	 */
+	const char *dir;
 };
 
 #define RUN_HOOKS_OPT_INIT { \