[abhinav]

Reading several objects separately starts one cat-file process per
object.
ReadObjects reads all requested hashes through one cat-file --batch
process instead.

Expose a tar.Reader-style ObjectBatchReader: Next returns object
metadata and skips any unread body, Read stops at that object's EOF,
and Close waits for completion or terminates an unfinished batch.
Object bodies are size-framed, so embedded newlines and NUL bytes
remain unchanged.
Use LF-delimited headers for compatibility with Git 2.38.

[abhinav]

This change is part of the following stack:

• git: Add streaming batch object reads #1480 ◀
  • storage: Add batch reads to the backend contract #1481
    • spice: Batch branch state and head loading #1489
      • log: Reuse loaded branch state in listings #1490

_{Change managed by git-spice.}