Add case insensitivity support when using git ls-files

Add case insensitivity support when using git ls-files

When mydir/filea.txt is added, mydir/ is renamed to MyDir/, and
MyDir/fileb.txt is added, running git ls-files mydir only shows
mydir/filea.txt. Running git ls-files MyDir shows MyDir/fileb.txt.
Running git ls-files mYdIR shows nothing.

With this patch running git ls-files for mydir, MyDir, and mYdIR shows
mydir/filea.txt and MyDir/fileb.txt.

Wildcards are not handled case insensitively in this patch. Example:
MyDir/aBc/file.txt is added. git ls-files MyDir/a* works fine, but git
ls-files mydir/a* does not.

Signed-off-by: Joshua Jensen <jjensen@workspacewhiz.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>