The latest commit cache is currently used only for listing tree files.
However, a cold start may take longer than directly invoking the Git
command. This PR addresses the issue of slow response times when
accessing raw files, improving performance in such scenarios.
```log
gitea.log:105521:2024/12/23 08:22:18 ...eb/routing/logger.go:68:func1() [W] router: slow GET /xxxx/xxxxxx/raw/commit/xxxxxxxxxxxxxxxxxxxxxxxxxxx/.editorconfig for 172.18.0.5:53252, elapsed 3526.8ms @ repo/download.go:117(repo.SingleDownload)
```
Fixes#29101
Related #29298
Discard all read data to prevent misinterpreting existing data. Some
discard calls were missing in error cases.
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Co-authored-by: yp05327 <576951401@qq.com>
