chunked: fix unmarshaling of file names

[giuseppe]

The getString() function was used to extract string values, but it doesn't handle escaped characters. Replace it with iter.ReadString() that is slower but handles escaped characters correctly.

Closes: https://github.com/containers/storage/issues/1878

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[rhatdan]

LGTM @nalind @mtrmac @saschagrunert @flouthoc PTAL

[giuseppe]

• (I am ambivalent about the concept of StringsBuf, there is no documented rationale. Is it to save on per-object allocation cost?)

yeah it was done exactly for that reason, but now that we are not using for name is unclear if it is still worth having.

For now, I've simplified the implementation and dropped the StringsBuf performance hack, we can revisit later if needed

[mtrmac]

(Just to be explicit, I’m completely fine with StringsBuf continuing to exist. I have no reason to prefer either way.)

[mtrmac]

/lgtm