Batch phpcs calls into a single invocation

[sirbrillig]

Fixes #115

Summary

• Replaces the 2N per-file phpcs launches in runGitWorkflow and runSvnWorkflow with a single phpcs invocation that scans all file versions at once
• All modified and unmodified file contents are written to a temp directory (/tmp/phpcs-changed-XXXX/new/… and /tmp/phpcs-changed-XXXX/old/…) and phpcs runs once across all of them, eliminating startup overhead that previously scaled linearly with the number of changed files
• The per-file library functions runGitWorkflowForFile and runSvnWorkflowForFile are unchanged

This is a different approach to #114

Approach

Each workflow now has three phases:

1. Pre-batch — check caches, determine which files are new (no unmodified version), collect the lists of files that need scanning
2. Batch — single getPhpcsOutputForGitBatch / getPhpcsOutputForSvnBatch call writes all content to a temp dir and runs phpcs once; results are cached individually per file
3. Filter — parse results and compute new messages per file as before

Trade-off

The batch approach always scans the unmodified version of uncached files even when the modified version has no messages. Previously the per-file path skipped the unmodified scan in that case. The trade-off is intentional: one saved phpcs startup cost (≥250 ms) outweighs the cost of a few extra file sniffs.

Performance

Measured with benchmark.sh on 10 staged PHP files (PSR2 standard, 10 runs):

	Mean	Relative
trunk: 20 phpcs calls	5.74 s ± 0.11 s	1.00×
batch (this branch): 1 phpcs call	2.44 s ± 0.05 s	2.36× faster

Changes

• PhpcsChanged/ShellOperator.php — add getPhpcsOutputForGitBatch and getPhpcsOutputForSvnBatch to the interface
• PhpcsChanged/UnixShell.php — implement both batch methods using a temp directory and a single phpcs invocation; private helpers writeTempFile, runBatchPhpcs, cleanupTempDir
• tests/helpers/TestShell.php — override both batch methods to delegate to existing per-file mocks (shell redirections can't be simulated in tests)
• PhpcsChanged/Cli.php — rewrite runGitWorkflow and runSvnWorkflow with the three-phase approach
• tests/GitWorkflowTest.php / tests/SvnWorkflowTest.php — update tests affected by the always-scan-unmodified trade-off; add multi-file batch tests verifying new-file handling and cache hit behaviour