Teach git-p4 to ignore case in perforce filenames if configured.

Teach git-p4 to ignore case in perforce filenames if configured.

When files are added to perforce, the path to that file has whichever case
configuration that exists on the machine of the user who added the file.
What does that mean? It means that when Alice adds a file

//depot/DirA/FileA.txt

... and Bob adds:

//depot/dirA/FileB.txt

... we may or may not get a problem. If a user sets the config variable
git-p4.ignorecase to "true", we will consider //depot/DirA and //depot/dirA
to be the same directory.

Signed-off-by: Tor Arvid Lund <torarvid@gmail.com>
Acked-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>