Truncate DocumentFile before writing to it

Before 43d88fd, this was not needed, because the file was always deleted
and re-created first.

This should resolve issues orgzly-revived#385 and orgzly-revived#395.

app/src/main/java/com/orgzly/android/repos/DocumentRepo.java

@@ -194,7 +194,7 @@ public VersionedRook storeBook(File file, String repoRelativePath) throws IOExce
             }
         }
 
-        try (OutputStream out = context.getContentResolver().openOutputStream(destinationFile.getUri())) {
+        try (OutputStream out = context.getContentResolver().openOutputStream(destinationFile.getUri(), "wt")) {
             MiscUtils.writeFileToStream(file, out);
         }