unpack-trees: fix nested sparse-dir search

unpack-trees: fix nested sparse-dir search

The iterated search in find_cache_entry() was recently modified to
include a loop that searches backwards for a sparse directory entry that
matches the given traverse_info and name_entry. However, the string
comparison failed to actually concatenate those two strings, so this
failed to find a sparse directory when it was not a top-level directory.

This caused some errors in rare cases where a 'git checkout' spanned a
diff that modified files within the sparse directory entry, but we could
not correctly find the entry.

Helped-by: Johannes Schindelin <redacted>
Helped-by: René Scharfe <redacted>
Signed-off-by: Derrick Stolee <redacted>
Reviewed-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>