git-gui: Use a smaller pipe buffer for update-index.

git-gui: Use a smaller pipe buffer for update-index.

When we shove a large number of files at update-index and they have
very short path names we are likely going to fit a large number of
them into the pipe buffer very early; thereby seeing a huge progress
update followed by lots of waiting between progress updates due to
the latency of update-index.

Using a smaller buffer should help smooth out the progress updates
as we are better able to keep tabs on the update-index process'
progress through our list of paths.

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