On Windows, mkdir -p and add_file/add_dir key implicit parent directories with \ #2830

Version: 0.20.3, InMemoryFs, Windows host. Follow-up to #2425 / #2435.

Repro

let fs = Arc::new(InMemoryFs::new());
let mut sh = Bash::builder().fs(fs.clone()).cwd("/tmp").build();
sh.exec("mkdir -p a/b; mkdir c").await?;
// fs.snapshot() paths on Windows: "/tmp\a", "/tmp\a\b", "/tmp/c"

Lookups still work, because they normalize. But snapshot() exposes the \ keys, so a caller that maps snapshot paths back to / paths loses every directory that mkdir -p created.

Cause

src/fs/memory.rs builds parent paths with current.push(component) in four places:

• add_file (line 935)
• add_dir (line 1023)
• add_lazy_file (line 1101)
• recursive mkdir (line 1486)

PathBuf::push adds the host separator. #2435 closed this class for Path::join and rated the remaining push sites "normalized downstream — consistency, not a live bug". Here, though, the pushed path becomes an entry-map key directly. That's the same shape as the InMemoryFs::rename fan-out the PR fixed.

Fix

Use the helper the file already imports, at all four sites:

-                current.push(component);
+                current = vfs_join(&current, component);

Tested against 0.20.3 on Windows: cargo test --lib fs:: passes 224/224, and our downstream filesystem tests pass 10/10 with this patch in place of our workaround.