[RFC] Case insensitive Git attributes

Hi,

Git attributes for path names are generally case sensitive. However, on 
a case insensitive file system (e.g. macOS/Windows) they appear to be
case insensitive (`*.bar` would match `foo.bar` and `foo.BAR`). That 
works great until a Git users joins the party with a case sensitive file 
system. For this Git user only files that match the exact case of the 
attribute pattern get the attributes (only `foo.bar`).

This inconsistent behavior can confuse Git users. An advanced Git user
could use a glob pattern (e.g. `*.[bB][aA][rR]) to match files in a
case insensitive way. However, this can get confusing quickly, too.

I wonder if we can do something about this. One idea could be to add an
attribute "case-sensitive" (or "caseSensitive") and set it to false 
(if desired) for all files in .gitattributes for a given repo.

### .gitattributes example ###

* case-sensitive=false
*.bar something

###

I haven't looked into the feasibility of an implementation, yet. However,
would that be an acceptable approach?

Thanks,
Lars