Only split shell lines which don't have newlines

[Chris Warburton]

We're splitting long lines outputted by shell commands, to prevent Emacs
from hanging. We do this by taking 1000 character chunks of the input
and putting newlines in between.

This ignores the fact that these chunks might themselves contain
newlines.

We should check each chunk to see whether it contains any newlines; if
not, we concat with a newline like we do at the moment. If it does
contain a newline, then we just concat it as-is.

The worst case scenario is when one chunk starts with a newline; in that
case, there will be a 999 character run without any newlines, followed
by another 1000-character chunk which may not contain any newlines. I
think Emacs should cope with 1999 character lines like this; it's mostly
when a line gets to several MB that things start to go wrong.