command: add helper function to split property paths

We've just checked whether a sub-path started with "name/", but that
changes behavior whether the property name has a trailing '/' or not.
Using a helper function to split of path components avoids this problem.

options/m_property.c

@@ -232,6 +232,20 @@ int m_property_do(struct mp_log *log, const m_option_t *prop_list,
     }
 }
 
+bool m_property_split_path(const char *path, bstr *prefix, char **rem)
+{
+    char *next = strchr(path, '/');
+    if (next) {
+        *prefix = bstr_splice(bstr0(path), 0, next - path);
+        *rem = next + 1;
+        return true;
+    } else {
+        *prefix = bstr0(path);
+        *rem = "";
+        return false;
+    }
+}
+
 static int m_property_do_bstr(const m_option_t *prop_list, bstr name,
                               int action, void *arg, void *ctx)
 {

options/m_property.h

@@ -122,6 +122,12 @@ enum mp_property_return {
 int m_property_do(struct mp_log *log, const struct m_option* prop_list,
                   const char* property_name, int action, void* arg, void *ctx);
 
+// Given a path of the form "a/b/c", this function will set *prefix to "a",
+// and rem to "b/c", and return true.
+// If there is no '/' in the path, set prefix to path, and rem to "", and
+// return false.
+bool m_property_split_path(const char *path, bstr *prefix, char **rem);
+
 // Print a list of properties.
 void m_properties_print_help_list(struct mp_log *log,
                                   const struct m_option* list);

player/command.c

@@ -853,16 +853,19 @@ static int tag_property(m_option_t *prop, int action, void *arg,
     }
     case M_PROPERTY_KEY_ACTION: {
         struct m_property_action_arg *ka = arg;
-        bstr key = bstr0(ka->key);
-        if (bstr_eatstart0(&key, "list/")) {
+        bstr key;
+        char *rem;
+        m_property_split_path(ka->key, &key, &rem);
+        if (bstr_equals0(key, "list")) {
             struct m_property_action_arg nka = *ka;
-            nka.key = key.start; // ok because slice ends with \0
+            nka.key = rem;
             return m_property_read_list(action, &nka, tags->num_keys,
                                         get_tag_entry, tags);
         }
         // Direct access without this prefix is allowed for compatibility.
-        bstr_eatstart0(&key, "by-key/");
-        char *meta = mp_tags_get_bstr(tags, key);
+        bstr k = bstr0(ka->key);
+        bstr_eatstart0(&k, "by-key/");
+        char *meta = mp_tags_get_bstr(tags, k);
         if (!meta)
             return M_PROPERTY_UNKNOWN;
         switch (ka->action) {