git-gui: Correctly handle CR vs. LF within the console of fetch.

git-gui: Correctly handle CR vs. LF within the console of fetch.

Because the remote end is likely to send us progress meters by
resetting each line with a CR (and no LF) we should display those
meters by replacing the last line of text with the next line,
just like a normal xterm would do.

This makes the output of fetch look about the same as if we ran it
from within an xterm.

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