using wildcard does not use cache with multistage build

[manishtomar]

Steps to reproduce:

1. Unzip buildkitbug.zip and cd buildkitbug
2. docker build -t test .
3. echo anything > conf/sample.yaml
4. docker build -t test .

Expected results:

Since only conf directory is changed I expect first stage (builder) to be completely cached when run again instead COPY and RUN commands are run. This does not happen if COPY does not use wildcard, i.e.COPY main.go /app instead of COPY *.go /app. This problem does not exist in legacy builder.

[nphmuller]

This is currently blocking us from using buildkit, because it destroys build performance.