git-p4: honor core.ignorecase when using P4 client specs

git-p4: honor core.ignorecase when using P4 client specs

Perforce depot may record paths in mixed cases, e.g. "p4 files" may
show that there are these two paths:

   //depot/Path/to/file1
   //depot/pATH/to/file2

and with "p4" or "p4v", these end up in the same directory, e.g.

   //depot/Path/to/file1
   //depot/Path/to/file2

which is the desired outcome on case insensitive systems.

If git-p4 is used with client spec "//depot/Path/...", however, then
all files not matching the case in the client spec are ignored (in
the example above "//depot/pATH/to/file2").

Fix this by using the path case that appears first in lexicographical
order when core.ignorecase is set to true. This behavior is consistent
with "p4" and "p4v".

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Acked-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>