git-gui: Correct ls-tree buffering problem in browser
Our file browser was showing bad output as it did not properly buffer
a partial record when read from `ls-tree -z`. This did not show up on
my Mac OS X system as most trees are small, the pipe buffers generally
big and `ls-tree -z` was generally fast enough that all data was ready
before Tcl started to read. However on my Cygwin system one of my
production repositories had a large enough tree and packfile that it
took a couple of pipe buffers for `ls-tree -z` to complete its dump.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>