Refactor iterate git tree to reduce memory usage

[lunny]

Make git tree recursive iteration stoppable and reduce the memory usage when calculating language stats.

[wxiaoguang]

It doesn't seem right, especially the different "ctx" are mixed together.

And in the end, you still read everything into memory entries = append(entries, entry)

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To make it overall right, it needs to consume the outputs on the fly, and make the git command has ability to limit the read data size.

[lunny]

It doesn't seem right, especially the different "ctx" are mixed together.

And in the end, you still read everything into memory entries = append(entries, entry)

To make it overall right, it needs to consume the outputs on the fly, and make the git command has ability to limit the read data size.

This PR introduced an iteration method and only reduces memory when calculating language stats. For tree list, they are kept as before and we can improve it in the future.