mu: Change default max msg size to 500 Mb

Seems people are getting really big mails these days, so let's up the
default (which is also what mu4e uses) to 500 Mb (which should be enough
for everyone, always)

lib/mu-index.c

@@ -37,7 +37,9 @@
 #include "mu-util.h"
 
 #define	MU_LAST_USED_MAILDIR_KEY "last_used_maildir"
-#define MU_INDEX_MAX_FILE_SIZE (50*1000*1000) /* 50 Mb */
+#define MU_INDEX_MAX_FILE_SIZE (500*1000*1000) /* 500 Mb */
+/* apparently, people are getting really big mails, so let us index those (by
+ * default)*/
 
 struct _MuIndex {
 	MuStore		*_store;