fsck: avoid misleading variable name

fsck: avoid misleading variable name

When reporting a problem, `git fsck` emits a message such as:

    missing blob 1234abcd (:file)

However, this can be ambiguous when the problem is detected in the index
of a worktree other than the one in which `git fsck` was invoked. To
address this shortcoming, 592ec63b38 (fsck: mention file path for index
errors, 2023-02-24) enhanced the output to mention the path of the index
when the problem is detected in some other worktree:

    missing blob 1234abcd (.git/worktrees/wt/index:file)

Unfortunately, the variable in fsck_index() which controls whether the
index path should be shown is misleadingly named "is_main_index" which
can be misunderstood as referring to the main worktree (i.e. the one
housing the .git/ repository) rather than to the current worktree (i.e.
the one in which `git fsck` was invoked). Avoid such potential confusion
by choosing a name more reflective of its actual purpose.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>