Only run the merge tool on files with differences

If the backup file and the data file are equal, there is no need to run
the merge tool.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -781,6 +781,7 @@ void io_stop_psave_thread(void);
 void io_set_lock(void);
 unsigned io_dump_pid(char *);
 unsigned io_get_pid(char *);
+int io_files_equal(const char *, const char *);
 int io_file_is_empty(char *);
 int io_file_cp(const char *, const char *);
 void io_unset_modified(void);