tidb-lightning: Inconsistent handling and checking of log file name

I noticed this when troubleshooting an issue where TiDB Lightning logging was misbehaving under TiDB Operator. TiDB Operator invokes tidb-lightning with --log-file="" (https://github.com/pingcap/tidb-operator/blob/master/charts/tidb-lightning/templates/scripts/_start_lightning.sh.tpl#L53) as well as lightning.file = "-" in the config file (https://github.com/pingcap/tidb-operator/blob/master/charts/tidb-lightning/values.yaml#L115).

It seems like tidb-lightning treats these differently? Or doesn't check them properly when setting up logging? In any case, in the following file one line uses "globalCfg.App.File" and a couple lines later "globalCfg.App.Config.File" is used instead:

https://github.com/pingcap/tidb/blob/cb8e65abcbd873b71ff81039ebc31b619830897d/br/cmd/tidb-lightning/main.go#L34

The result of this in the case of TiDB Operator seems to be that it outputs the line "Verbose debug logs will be written to" without a filename.

Start in br/cmd/tidb-lightning/main.go at the logging setup around the referenced lines, comparing uses of globalCfg.App.File and globalCfg.App.Config.File. Run tidb-lightning with --log-file="" and lightning.file="-" to observe the logging behavior. Done means the two inputs are handled consistently and the verbose debug message does not omit its filename.

Written by the indexing model from the issue text.