Reuse the line buffer between strings in PacketLineIn

When reading pkt-lines off an InputStream we are quite likely to
consume a whole group of fairly short lines in rapid succession, such
as in the have exchange that occurs in the fetch-pack/upload-pack
protocol.  Rather than allocating a throwaway buffer for each
line's raw byte sequence, reuse a buffer that is equal to the small
side-band packet size, which is 1000 bytes.  Text based pkt-lines
are required to be less than this size because many widely deployed
versions of C Git use a statically allocated array of this length.

Change-Id: Ia5c8e95b85020f7f80b6d269dda5059b092d274d
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>