git-gui: Honor system encoding for filenames.

git-gui: Honor system encoding for filenames.

Since git operates on filenames using the operating system encoding
any data we are receiving from it by way of a pipe, or sending to it
by way of a pipe must be formatted in that encoding.  This should
be the same as the Tcl system encoding, as its the encoding that
applications should be using to converse with the operating system.

Sadly this does not fix the gitweb/test file in git.git on Macs;
that's due to something really broken happening in the filesystem.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>