Fix relative input paths being silently skipped

rsync reads --files-from entries relative to the source directory
('/'), so relative inputs pointed to the wrong path and were ignored
due to --ignore-missing-args. Convert inputs to absolute paths.

simple_backup/simple_backup.py

@@ -508,7 +508,8 @@ class Backup:
                 if not os.path.exists(i):
                     logger.warning('Input %s not found. Skipping', i)
                 else:
-                    fp.write(i)
+                    # rsync reads --files-from paths relative to the source ('/')
+                    fp.write(os.path.abspath(i))
                     fp.write('\n')
                     count += 1