Fix retries in git-cvsimport

Fix retries in git-cvsimport

Fixed a couple of bugs in recovering from broken connections:

The _line() method now returns undef correctly when the connection
is broken instead of falling off the function and returning garbage.

Retries are now reported to stderr and the eventual partially
downloaded file is discarded instead of being appended to.

The "Server gone away" test has been removed, because it was
reachable only if the garbage return bug bit.

Signed-off-by: Martin Mares <mj@ucw.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>