fix(git): filter git show <rev>:<path> blobs instead of passthrough

[MannXo]

Summary

• git show <rev>:<path> prints a file at a revision (a file read), but it was routed to passthrough and emitted uncompressed at 0% savings, while the <rev> -- <path> diff form already compacts.
• Route the blob form through the same filtering rtk read applies: minimal comment/blank-line stripping with language detected from the path extension, capped at the same 500-line default the diff form uses, guarded by never_worse so small files pass through unchanged.
• git show --stat and --format variants keep their previous behavior.

Fixes #3116

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test (2444 lib tests pass, zero clippy warnings)
• New unit tests in src/cmds/git/git.rs: blob-path extraction, comment stripping, 500-line cap with truncation marker, unknown-extension faithfulness, never_worse on tiny input, and a ≥60% savings check
• Manual testing against a real repo:
  • rtk git show HEAD:Cargo.toml — small file, byte-identical to raw (faithful)
  • rtk git show HEAD:src/cmds/git/git.rs — 3325-line file capped to 500 lines with a [2692 more lines] marker

[CLAassistant]

All committers have signed the CLA.

[KuSh]

Thanks for digging into this. The blob form is filtered on develop now: #3265 (merged 2026-09-11) routes git show <rev>:<path> into a size-capped preview window instead of raw passthrough, so the 0% savings case you reported is gone. Measured on current develop, a 259 KB blob comes back as 8 KB. The merged version keeps raw bytes rather than stripping comments, deliberately, so that non-UTF-8 and binary blobs survive the window, and it also handles the --stat and --format combinations you left on the old path. Closing as covered, but please comment here or reopen if you hit a blob shape the new path still misses.