Incorrect suffixMatch optimization of `**`

https://github.com/moby/patternmatcher/commit/2d3e8372017c5e8113b281ac3c2d6058101742ed introduced matchType to speed up matching; that changed semantics of **.

https://docs.docker.com/build/concepts/context/#matching says ** … matches any number of directories (including zero)”. (https://git-scm.com/docs/gitignore is even more restrictive).

But consider these added test cases:

diff --git a/patternmatcher_test.go b/patternmatcher_test.go
index 5f02521..81d73fd 100644
--- a/patternmatcher_test.go
+++ b/patternmatcher_test.go
@@ -184,6 +184,14 @@ func TestMatches(t *testing.T) {
                {"a.|)$(}+{bc", "a.|)$(}+{bc", true},
                {"dist/proxy.py-2.4.0rc3.dev36+g08acad9-py3-none-any.whl", "dist/proxy.py-2.4.0rc3.dev36+g08acad9-py3-none-any.whl", true},
                {"dist/*.whl", "dist/proxy.py-2.4.0rc3.dev36+g08acad9-py3-none-any.whl", true},
+               {"**.md", ".md", true},
+               {"**.md", "name.md", true},
+               {"**.md", "dir/.md", true},
+               {"**.md", "dir/name.md", true},
+               {"**.m[dD]", ".md", true},
+               {"**.m[dD]", "name.md", false},
+               {"**.m[dD]", "dir/.md", true},
+               {"**.m[dD]", "dir/name.md", false},
        }
        multiPatternTests := []multiPatternTestCase{
                {[]string{"**", "!util/docker/web"}, "util/docker/web/foo", false},

The suffixMatch code path is a simple suffix, so name.md and dir/name.md match; the regexpMatch code path uses (.*/)?, so name.md and dir/name.md don’t match.

(On the other side of **, matching is also not interpreting the value as “any number of directories”, d** matches dir/name.md, but at least it is consistent between prefixMatch and regexpMatch.)

Start with patternmatcher_test.go and the added cases for **.md and **.m[dD], then trace the matchType, suffixMatch, and regexpMatch paths. Run the Go tests to reproduce the differing results. Done means ** matches any number of directories, including zero, consistently across these cases.