This follows
* https://github.com/go-gitea/gitea/issues/18553
Introduce `RunWithContextString` and `RunWithContextBytes` to help the refactoring. Add related unit tests. They keep the same behavior to save stderr into err.Error() as `RunInXxx` before.
Remove `RunInDirTimeoutPipeline` `RunInDirTimeoutFullPipeline` `RunInDirTimeout` `RunInDirTimeoutEnv` `RunInDirPipeline` `RunInDirFullPipeline` `RunTimeout`, `RunInDirTimeoutEnvPipeline`, `RunInDirTimeoutEnvFullPipeline`, `RunInDirTimeoutEnvFullPipelineFunc`.
Then remaining `RunInDir` `RunInDirBytes` `RunInDirWithEnv` can be easily refactored in next PR with a simple search & replace:
* before: `stdout, err := RunInDir(path)`
* next: `stdout, _, err := RunWithContextString(&git.RunContext{Dir:path})`
Other changes:
1. When `timeout <= 0`, use default. Because `timeout==0` is meaningless and could cause bugs. And now many functions becomes more simple, eg: `GitGcRepos` 9 lines to 1 line. `Fsck` 6 lines to 1 line.
2. Only set defaultCommandExecutionTimeout when the option `setting.Git.Timeout.Default > 0`
