Right now, there's quite a lot of different function calls to determine
whether a path component matches a specific name after normalization
from the filesystem. We have a function for each of {gitattributes,
gitmodules, gitignore} multiplicated with {generic, NTFS, HFS} checks.
In the long time, this is unmaintainable in case there are e.g. new
filesystems with specific semantics, blowing up the number of functions
we need to implement.

Replace all functions with a simple `git_path_is_gitfile` function,
which accepts an enum pointing out the filename that is to be checked
against as well as the filesystem normalizations to check for. This
greatly simplifies implementation at the expense of the caller having to
invoke a somewhat longer function call.