mu: add '--lazy-check' option for indexing

Add an option --lazy-check to ignore any directories that don't have
their ctime changed since the last indexing operation.

There are a few corner-cases (such as editing a message outside mu's
control) where this might miss a change, but apart from that, makes
indexing in for a maildir (and its sub-maildirs) almost a no-op if there
were no changes.

mu/mu-config.h

@@ -112,6 +112,8 @@ struct _MuConfig {
 	gboolean        rebuild;	/* empty the database before indexing */
 	gboolean        autoupgrade;    /* automatically upgrade db
 					 * when needed */
+	gboolean        lazycheck;      /* don't check dirs with up-to-date
+					 * timestamps */
 	int             xbatchsize;     /* batchsize for xapian
 					 * commits, or 0 for
 					 * default */