fetch: introduce machine-parseable "porcelain" output format

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 and new 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.

A notable ommission here is that the output format does not include the
remote from which a reference was fetched, which might be important
information especially in the context of multi-remote fetches. But as
such a format would require us to print the remote for every single
reference update due to parallelizable fetches it feels wasteful for the
most likely usecase, which is when fetching from a single remote.

In a similar spirit, a second restriction is that this cannot be used
with `--recurse-submodules`. This is because any reference updates would
be ambiguous without also printing the repository in which the update
happens.

Considering that both multi-remote and submodule fetches are user-facing
features, using them in conjunction with `--porcelain` that is intended
for scripting purposes is likely not going to be useful in the majority
of cases. With that in mind these restrictions feel acceptable. If
usecases for either of these come up in the future though it is easy
enough to add a new "porcelain-v2" format that adds this information.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>