Implement fs.caseInsensitive in .gitattributes

[Denton-L]

This feature should make it easier for our case-sensitive and non-case-sensitive friends to work together.

If this option is enabled in the .gitattributes file, it should not allow a user on a case-sensitive filesystem to add files to the index that may collide on a non-case-sensitive system (e.g. asdf and Asdf).

Also, we should warn or die when creating a merge that causes a name collision to happen.

Perhaps, this could be accompanied by a config setting too.