midx: disallow running outside of a repository

The multi-pack-index command supports working with arbitrary object
directories via the `--object-dir` flag. Though this has historically
worked in arbitrary repositories (including when the command itself was
run outside of a Git repository), this has been somewhat of an accident.

For example, running:

    git multi-pack-index write --object-dir=/path/to/repo/objects

outside of a Git repository causes a BUG(). This is because the
top-level `cmd_multi_pack_index()` function stops parsing when it sees
"write", and then fills in the default object directory (the result of
calling `get_object_directory()`) before handing off to
`cmd_multi_pack_index_write()`. But there is no repository to
initialize, and so calling `get_object_directory()` results in a BUG()
(indicating that the current repository is not initialized).

Another case where this doesn't quite work as expected is when operating
in a SHA-256 repository. To see the failure, try this in your shell:

    git init --object-format=sha256 repo
    git -C repo commit --allow-empty base
    git -C repo repack -d

    git multi-pack-index --object-dir=$(pwd)/repo/.git/objects write

and observe that we cannot open the `.idx` file in "repo", because the
outermost process assumes that any repository that it works in also uses
the default value of `the_hash_algo` (at the time of writing, SHA-1).

There may be compelling reasons for trying to work around these bugs,
but working in arbitrary `--object-dir`'s is non-standard enough (and
likewise, these bugs prevalent enough) that I don't think any workflows
would be broken by abandoning this behavior.

Accordingly, restrict the `multi-pack-index` builtin to only work when
inside of a Git repository (i.e., its main utility becomes selecting
which alternate to operate in), which avoids both of the bugs above.

(Note that you can still trigger a bug when writing a MIDX in an
alternate which does not use the same object format as the repository
which it is an alternate of, but that is an unrelated bug to this one).

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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