add pass_filenames_via_stdin for large changesets

pre-commit currently passes selected filenames to hooks via argv.
For large changesets (or --all-files), argv length limits are hit and
filenames are partitioned, causing multiple hook invocations.

This means there is currently no built-in way to pass filenames to an
underlying hook in one shot without chunking / re-running. The only practical
workaround is to set pass_filenames: false and run custom git operations in
hook code to reconstruct the file set, which is expensive and duplicates
pre-commit's own file-selection logic.

This change adds a hook option:

    pass_filenames_via_stdin: true

When enabled, pre-commit sends filenames as NUL-delimited bytes on stdin and
runs the hook in a single invocation (no argv chunking).

Why NUL-delimited stdin:
- safe for filenames containing spaces/newlines
- matches established -0 conventions in unix tooling

Usage for hook authors:
- shell:

    while IFS= read -r -d '' filename; do
        ...
    done

- python:

    data = sys.stdin.buffer.read()
    filenames = [os.fsdecode(p) for p in data.split(b'\0') if p]

Behavior notes:
- default remains argv-based passing
- pass_filenames: false still disables filename passing entirely

Implementation includes schema/runtime wiring, shared NUL encode/decode
helpers, and tests covering defaulting and runtime behavior.

pre_commit/clientlib.py

@@ -256,6 +256,7 @@ MANIFEST_HOOK_DICT = cfgv.Map(
     cfgv.Optional('always_run', cfgv.check_bool, False),
     cfgv.Optional('fail_fast', cfgv.check_bool, False),
     cfgv.Optional('pass_filenames', cfgv.check_bool, True),
+    cfgv.Optional('pass_filenames_via_stdin', cfgv.check_bool, False),
     cfgv.Optional('description', cfgv.check_string, ''),
     cfgv.Optional('language_version', cfgv.check_string, C.DEFAULT),
     cfgv.Optional('log_file', cfgv.check_string, ''),