Fix panic in git pktline splitter

This bug was discovered during work on !844 (merged)

Within the Git pkt-line format, there are four special empty packets: 0000, 0001, 0002, 0003. Of these four our splitter only handled 0000 correctly. The others would lead to a slice out of bounds panic. This change fixes that so that none of the four special packets blows up the parser.

Because we had more than one pkt-line parser, this change also factors out the parser into a package so we can reuse it.