fetch: introduce machine-parseable "porcelain" output format

The output of git-fetch(1) is obviously designed for consumption by users, only: we neatly columnize data, we abbreviate reference names, we print neat arrows and we don't provide information about actual object IDs that have changed. This makes the output format basically unusable in the context of scripted invocations of git-fetch(1) that want to learn about the exact changes that the command performs.

Introduce a new machine-parseable "porcelain" output format that is supposed to fix this shortcoming. This output format is intended to provide information about every reference that is about to be updated, the old object ID that the reference has been pointing to and the new object ID it will be updated to. Furthermore, the output format provides the same flags as the human-readable format to indicate basic conditions for each reference update like whether it was a fast-forward update, a branch deletion, a rejected update or others.

The output format is quite simple:

<flag> <old-object-id> <new-object-id> <local-reference>\n

We assume two conditions which are generally true:

- The old an dnew object IDs have fixed known widths and cannot
  contain spaces.

- References cannot contain newlines.

With these assumptions, the output format becomes unambiguously parseable. Furthermore, given that this output is designed to be consumed by scripts, the machine-readable data is printed to stdout instead of stderr like the human-readable output is. This is mostly done so that other data printed to stderr, like error messages or progress meters, don't interfere with the parseable data.

Closes #147 (closed).