Resolve "Blob#batch can't handle frozen string paths"

What does this MR do?

Fixes runtime errors related to modifying frozen strings

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Gitaly calls return frozen strings. Some rugged calls attempt to modify strings which are the outputs of gitaly calls, with some combinations of feature flags.

Modifying the string is unnecessary, and poor form anyway in a library method, so we can just switch to sub.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• [-] Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #46498 (closed)