globset: permit ** to appear anywhere

Previously, `man gitignore` specified that `**` was invalid unless it
was used in one of a few specific circumstances, i.e., `**`, `a/**`,
`**/b` or `a/**/b`. That is, `**` always had to be surrounded by either
a path separator or the beginning/end of the pattern.

It turns out that git itself has treated `**` outside the above contexts
as valid for quite a while, so there was an inconsistency between the
spec `man gitignore` and the implementation, and it wasn't clear which
was actually correct.

@okdana filed a bug against git[1] and got this fixed. The spec was wrong,
which has now been fixed [2] and updated[2].

This commit brings ripgrep in line with git and treats `**` outside of
the above contexts as two consecutive `*` patterns. We deprecate the
`InvalidRecursive` error since it is no longer used.

Fixes #373, Fixes #1098

[1] - https://public-inbox.org/git/C16A9F17-0375-42F9-90A9-A92C9F3D8BBA@dana.is
[2] - https://github.com/git/git/commit/627186d0206dcb219c43f8e6670b4487802a4921
[3] - https://git-scm.com/docs/gitignore

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