Monitor local git-upload-pack children for stderr output
If a locally forked git-upload-pack has issues it will print messages
to stderr, to tell us what went wrong. Rather than eating these we
display them on our own stderr (which may be buried in the eclipse log
file, but is at least still recorded) so we can better debug any issue
we may have.
We also use the thread that copies stderr->stderr to reap back the
child process since the API doesn't properly support that notion.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>