Any part of the library which asks the question can pass in the mode to have it
checked against `.gitmodules` being a symlink.

This is particularly relevant for adding entries to the index from the worktree
and for checking out files.

We may take in names from the middle of a string so we want the caller to let us
know how long the path component is that we should be checking.

These can't go into the public API yet as we don't want to introduce API or ABI
changes in a security release.

Given a path component it knows what to pass to the filesystem-specific
functions so we're protected even from trees which try to use the 8.3 naming
rules to get around us matching on the filename exactly.

The logic and test strings come from the equivalent git change.