Use the asyncronous function infrastructure in builtin-fetch-pack.c.

Use the asyncronous function infrastructure in builtin-fetch-pack.c.

We run the sideband demultiplexer in an asynchronous function.

Note that earlier there was a check in the child process that closed
xd[1] only if it was different from xd[0]; this test is no longer needed
because git_connect() always returns two different file descriptors
(see ec587fde0a76780931c7ac32474c8c000aa45134).

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>