box: make --box-commit-retries flag defaulting to 100 - Fixes #2054

Sometimes it takes many more commit retries than expected to commit a
multipart file, so split this number into its own config variable and
default it to 100 which should always be enough.

backend/box/upload.go

@@ -96,7 +96,9 @@ func (o *Object) commitUpload(SessionID string, parts []api.Part, modTime time.T
 	request.Attributes.ContentCreatedAt = api.Time(modTime)
 	var body []byte
 	var resp *http.Response
-	maxTries := fs.Config.LowLevelRetries
+	// For discussion of this value see:
+	// https://github.com/ncw/rclone/issues/2054
+	maxTries := o.fs.opt.CommitRetries
 	const defaultDelay = 10
 	var tries int
 outer: