path: refactor `repo_common_path()` family of functions

The functions provided by the "path" subsystem to derive repository
paths for the commondir, gitdir, worktrees and submodules are quite
inconsistent. Some functions have a `strbuf_` prefix, others have
different return values, some don't provide a variant working on top of
`strbuf`s.

We're thus about to refactor all of these family of functions so that
they follow a common pattern:

  - `repo_*_path()` returns an allocated string.

  - `repo_*_path_append()` appends the path to the caller-provided
    buffer while returning a constant pointer to the buffer. This
    clarifies whether the buffer is being appended to or rewritten,
    which otherwise wasn't immediately obvious.

  - `repo_*_path_replace()` replaces contents of the buffer with the
    computed path, again returning a pointer to the buffer contents.

The returned constant pointer isn't being used anywhere yet, but it will
be used in subsequent commits. Its intent is to allow calling patterns
like the following somewhat contrived example:

    if (!stat(&st, repo_common_path_replace(repo, &buf, ...)) &&
        !unlink(repo_common_path_replace(repo, &buf, ...)))
            ...

Refactor the commondir family of functions accordingly and adapt all
callers.

Note that `repo_common_pathv()` is converted into an internal
implementation detail. It is only used to implement `the_repository`
compatibility shims and will eventually be removed from the public
interface.

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

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