Merge branch 'lh/systemd-timers'

"git maintenance" scheduler learned to use systemd timers as a
possible backend.

* lh/systemd-timers:
  maintenance: add support for systemd timers on Linux
  maintenance: `git maintenance run` learned `--scheduler=<scheduler>`
  cache.h: Introduce a generic "xdg_config_home_for(…)" function

cache.h

@@ -1297,6 +1297,13 @@ int is_ntfs_dotmailmap(const char *name);
  */
 int looks_like_command_line_option(const char *str);
 
+/**
+ * Return a newly allocated string with the evaluation of
+ * "$XDG_CONFIG_HOME/$subdir/$filename" if $XDG_CONFIG_HOME is non-empty, otherwise
+ * "$HOME/.config/$subdir/$filename". Return NULL upon error.
+ */
+char *xdg_config_home_for(const char *subdir, const char *filename);
+
 /**
  * Return a newly allocated string with the evaluation of
  * "$XDG_CONFIG_HOME/git/$filename" if $XDG_CONFIG_HOME is non-empty, otherwise