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] - 627186d020
[3] - https://git-scm.com/docs/gitignore

globset/src/lib.rs

@@ -143,8 +143,13 @@ pub struct Error {
 /// The kind of error that can occur when parsing a glob pattern.
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub enum ErrorKind {
-    /// Occurs when a use of `**` is invalid. Namely, `**` can only appear
-    /// adjacent to a path separator, or the beginning/end of a glob.
+    /// **DEPRECATED**.
+    ///
+    /// This error used to occur for consistency with git's glob specification,
+    /// but the specification now accepts all uses of `**`. When `**` does not
+    /// appear adjacent to a path separator or at the beginning/end of a glob,
+    /// it is now treated as two consecutive `*` patterns. As such, this error
+    /// is no longer used.
     InvalidRecursive,
     /// Occurs when a character class (e.g., `[abc]`) is not closed.
     UnclosedClass,