Continue updates when pipe read has errno "Success"

Continue updates when pipe read has errno "Success"

When we are reading from a pipe and receive a signal, our
read call fails and ferror() returns true. The current
behavior is to call end_update and report failure. However,
we can detect this situation by checking that errno is set
to success and continue the reading process.

You can provoke this behavior by running a "tig blame" that
takes a few seconds and then resizing the terminal that tig
is running in (you should get an incomplete blame output and
the error "Failed to read: Success").

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Jonas Fonseca <fonseca@diku.dk>