fetch: print left-hand side when fetching HEAD:foo

fetch: print left-hand side when fetching HEAD:foo

`store_updated_refs()` parses the remote reference for two purposes:

    - It gets used as a note when writing FETCH_HEAD.

    - It is passed through to `display_ref_update()` to display
      updated references in the following format:

      ```
       * branch               master          -> master
      ```

In most cases, the parsed remote reference is the prettified reference
name and can thus be used for both cases. But if the remote reference is
HEAD, the parsed remote reference becomes empty. This is intended when
we write the FETCH_HEAD, where we skip writing the note in that case.
But when displaying the updated references this leads to inconsistent
output where the left-hand side of reference updates is missing in some
cases:

```
$ git fetch origin HEAD HEAD:explicit-head :implicit-head main
From https://github.com/git/git
 * branch                  HEAD       -> FETCH_HEAD
 * [new ref]                          -> explicit-head
 * [new ref]                          -> implicit-head
 * branch                  main       -> FETCH_HEAD
```

This behaviour has existed ever since the table-based output has been
introduced for git-fetch(1) via 165f390250 (git-fetch: more terse fetch
output, 2007-11-03) and was never explicitly documented either in the
commit message or in any of our tests. So while it may not be a bug per
se, it feels like a weird inconsistency and not like it was a concious
design decision.

The logic of how we compute the remote reference name that we ultimately
pass to `display_ref_update()` is not easy to follow. There are three
different cases here:

    - When the remote reference name is "HEAD" we set the remote
      reference name to the empty string. This is the case that causes
      the left-hand side to go missing, where we would indeed want to
      print "HEAD" instead of the empty string. This is what
      `prettify_refname()` would return.

    - When the remote reference name has a well-known prefix then we
      strip this prefix. This matches what `prettify_refname()` does.

    - Otherwise, we keep the fully qualified reference name. This also
      matches what `prettify_refname()` does.

As the return value of `prettify_refname()` would do the correct thing
for us in all three cases, we can thus fix the inconsistency by passing
through the full remote reference name to `display_ref_update()`, which
learns to call `prettify_refname()`. At the same time, this also
simplifies the code a bit.

Note that this patch also changes formatting of the block that computes
the "kind" (which is the category like "branch" or "tag") and "what"
(which is the prettified reference name like "master" or "v1.0")
variables. This is done on purpose so that it is part of the diff,
hopefully making the change easier to comprehend.

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