hook.[ch]: move find_hook() from run-command.c to hook.c

Move the find_hook() function from run-command.c to a new hook.c
library. This change establishes a stub library that's pretty
pointless right now, but will see much wider use with Emily Shaffer's
upcoming "configuration-based hooks" series.

Eventually all the hook related code will live in hook.[ch]. Let's
start that process by moving the simple find_hook() function over
as-is.

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

hook.h

@@ -0,0 +1,11 @@
+#ifndef HOOK_H
+#define HOOK_H
+
+/*
+ * Returns the path to the hook file, or NULL if the hook is missing
+ * or disabled. Note that this points to static storage that will be
+ * overwritten by further calls to find_hook and run_hook_*.
+ */
+const char *find_hook(const char *name);
+
+#endif