Fix GitRemote HEAD fetch

[adamziel]

What it does

Fixes GitRemote::pull( 'HEAD' ) / fetch( 'HEAD' ) so HEAD is resolved as the remote symbolic ref instead of being rewritten to refs/heads/HEAD.

Closes #292.

Rationale

Blueprints can use git:directory with ref: "HEAD". The PHP runtime treated that as a branch named HEAD, then failed because remotes advertise HEAD, not refs/heads/HEAD.

Implementation

• Requests the remote ref HEAD when the caller passes exactly HEAD.
• Writes a forced pull( 'HEAD' ) result to local HEAD, so GitFilesystem reads the fetched commit.
• Parses NUL-separated ls-refs attributes separately from ref names, including peeled: hashes.
• Adds regression tests for pulling HEAD from an in-memory Git endpoint and parsing NUL-separated ref attributes.

Testing instructions

vendor/bin/phpunit components/Git/Tests/
vendor/bin/phpcs -d memory_limit=1G --warning-severity=0 components/Git/class-gitremote.php components/Git/Tests/GitRemoteTest.php

Also manually checked ls_refs( 'HEAD' ) and shallow pull( 'HEAD' ) against https://github.com/woocommerce/wc-smooth-generator.git.