[BUG] Suspected with double asterisk in conditional include pattern

Hello,

I believe I've encountered a bug regarding the use of double asterisk ( /**/ ) within includeIf patterns.

git-config man pages state that

    **/ and /**, that can match multiple path components

They then refer to the gitignore man pages which further define supported wildcard patterns:

    A slash followed by two consecutive asterisks then a slash matches
zero or more directories.
    For example, "a/**/b" matches "a/b", "a/x/b", "a/x/y/b" and so on.

My understanding of these docs are that the pattern `/usr/local/**/Homebrew/` ought to match both`/usr/local/cache/Homebrew/foo` and `/usr/local/Homebrew/foo`.

However, given the following conditional include rule:

    [includeIf "gitdir:/usr/local/**/Homebrew/"]

The external config file is successfully included while in repo `/usr/local/cache/Homebrew/foo` but NOT `/usr/local/Homebrew/foo`.

If I change the pattern to `**/Homebrew/**`, then the pattern matches both desired repos. So it would seem that the `/**/` does not match 0 directories when the pattern begins with a slash.

Thank you, Jason