Fix nested .gitignore directory patterns not excluding under --exclude-gitignore

[Osamaali313]

Problem

matches_gitignore (mypy/modulefinder.py) decides whether to append a trailing / (needed for directory-only gitignore patterns like build/) using the wrong path:

def matches_gitignore(subpath: str, fscache: FileSystemCache, verbose: bool) -> bool:
    dir, _ = os.path.split(subpath)
    for gi_path, gi_spec in find_gitignores(dir):
        relative_path = os.path.relpath(subpath, gi_path)
        if fscache.isdir(relative_path):        # relative_path is relative to gi_path, not cwd
            relative_path = relative_path + "/"
        if gi_spec.match_file(relative_path):
            ...

relative_path is computed relative to the .gitignore's directory (gi_path) so it can be matched by gi_spec.match_file(...) — that part is correct (gitignore patterns are relative to the gitignore's location). But that same gi_path-relative string is then passed to fscache.isdir(...), which resolves paths relative to the process cwd. When the matched .gitignore lives in a scanned subdirectory (gi_path != cwd), the string doesn't resolve on disk, isdir returns False, the trailing / is never added, and directory-only patterns in nested .gitignores silently fail to exclude the directory.

Evidence (sibling proves intent)

The sibling matches_exclude in the same file does this correctly — it builds the match string via os.path.relpath(subpath) but calls isdir on the original path:

subpath_str = os.path.relpath(subpath).replace(os.sep, "/")
if fscache.isdir(subpath):        # original subpath, not the relativized string
    subpath_str += "/"

Fix

Stat the original subpath (keep relative_path only for the gitignore-spec match):

if fscache.isdir(subpath):
    relative_path = relative_path + "/"

Reproduction

subpath = "mypkg/sub" (a real directory), matched against a nested .gitignore in mypkg/ containing sub/:

	trailing / added?	directory excluded?
before (isdir(relative_path))	no (isdir("sub") is False from cwd)	no ❌
after (isdir(subpath))	yes	yes ✅

Reachable from find_sources.py (and modulefinder) whenever --exclude-gitignore is enabled and a nested .gitignore uses a directory-only pattern.

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[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[A5rocks]

Please make an issue first.

[Osamaali313]

Thanks — opened #21760 describing the bug (directory-only patterns in a nested .gitignore not excluded under --exclude-gitignore, because the trailing-slash isdir check stats a gitignore-relative path against cwd). This PR fixes it by statting the original subpath, matching the sibling matches_exclude.