git: parse sideband control code from pkt-lines
In Git's packfile transfer protocol[1], a pkt-line can start with \1, \2, or \3 to represent whether this line contains packfile data (1), progress information (2), or an error (3). Parse out this information as a field to make it easier for people debugging to filter to the pkt-lines of interest to them.
[1] https://www.kernel.org/pub/software/scm/git/docs/technical/pack-protocol.html
Related to #17093