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/xargs.py

@@ -142,7 +142,8 @@ def xargs(
     color: Make a pty if on a platform that supports it
     target_concurrency: Target number of partitions to run concurrently
     """
-    cmd_fn = cmd_output_p if color else cmd_output_b
+    use_pty = color and kwargs.get('input') is None
+    cmd_fn = cmd_output_p if use_pty else cmd_output_b
     retcode = 0
     stdout = b''
 
@@ -164,6 +165,8 @@ def xargs(
         _max_length = 8192 - len(cmd_exe) - len(' /c ') - 1024
 
     partitions = partition(cmd, varargs, target_concurrency, _max_length)
+    if kwargs.get('input') is not None and len(partitions) != 1:
+        raise AssertionError('`input` is only supported with one partition')
 
     def run_cmd_partition(
             run_cmd: tuple[str, ...],