Detect git worktrees and submodules in identify_vcs

[potiuk]

Closes #798.

Problem

In a linked git worktree (git worktree add ...), or inside a git submodule, .git is a regular file containing a gitdir: <path> pointer rather than a directory. flit/vcs/__init__.py:identify_vcs only checked .is_dir(), so in these contexts it returns None, flit/sdist.py:SdistBuilder.select_files silently falls back to the non-VCS file set (super().select_files() from flit_core), and the resulting sdist omits tracked files such as docs/, tests/, and anything declared through [tool.flit.sdist] include = [...]. No warning is emitted.

Change

Accept .git whether it is a directory or a file:

git_entry = p / '.git'
if git_entry.is_dir() or git_entry.is_file():
    return git

Downstream git ls-files naturally validates the pointer target — no extra parsing needed inside identify_vcs.

Test

Added tests/test_vcs.py::test_identify_git_worktree: create .git as a regular file containing a gitdir: pointer, identify_vcs now returns the git module. Existing test_identify_git_parent continues to cover the plain-checkout path.

Three other tests in tests/test_sdist.py and tests/test_wheel.py fail on current main in my environment — they also fail on main without this change, so they are unrelated and not regressed by this PR.

Context

Discovered during Apache Airflow provider release verification — 20 sdists diverged byte-for-byte from the released tarballs because the release was built from a plain checkout while the verifier ran from a worktree (apache/airflow#65771). The same failure shape surfaces when a worktree is bind-mounted into a container and the main repo's .git is not reachable.