PacketLineIn: Reuse internal lineBuffer for small strings

Most "ACK %s continue", "ACK %s common", "NAK" strings that are read
by the readACK() method and readString() are shorter than the
lineBuffer already available.  Reuse that buffer when reading from
the network stream and converting to a string with RawParseUtils to
avoid unnecessary temporary byte array allocations.

Change-Id: Ibc778d9f7721943a065041d80fc427ea50d90fff
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/transport/PacketLineIn.java

@@ -113,7 +113,12 @@ class PacketLineIn {
 		if (len == 0)
 			return "";
 
-		final byte[] raw = new byte[len];
+		byte[] raw;
+		if (len <= lineBuffer.length)
+			raw = lineBuffer;
+		else
+			raw = new byte[len];
+
 		IO.readFully(in, raw, 0, len);
 		if (raw[len - 1] == '\n')
 			len--;