`git rev-parse --is-inside-work-tree` and $GIT_WORK_TREE

So, I find this behaviour a little strange; I can't determine if it's a subtle bug, or intentionally undefined/‘fuzzy’ behaviour:

    $ cd a-repo/.git/
    $ pwd
    /path/to/a-repo/.git
    $ git rev-parse --is-inside-work-tree
    false
    $ export GIT_WORK_TREE=/path/to/a-repo
    $ git rev-parse --is-inside-work-tree
    true

i.e. when within the repository (the `.git` directory), and when that directory is a sub-directory of the working-tree, `rev-parse --is-inside-work-tree` reports *false* (reasonable enough, I suppose); but then if `$GIT_WORK_TREE` is set to precisely the directory that git was *already* assuming was the working-directory, then the same command, in the same location, reports *true*.

This should probably be made consistent: either `rev-parse --is-inside-work-tree` should report “true”, even inside the `.git` dir, as long as that directory is a sub-directory of the working-tree … or repository-directories / `$GIT_DIR` / `.git` directories should be excluded from truthy responses to `rev-parse --is-inside-work-tree`.

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