bake: preserve git subdir in remote bake context paths

[crazy-max]

The repro shown in #3462 was not correct and didn't handle subdir. With this change it supports both fragment and query format with a subdir.

$ docker buildx bake "https://github.com/tonistiigi/xx.git#refs/tags/v1.9.0" xx --print
#0 building with "default" instance using docker driver

#1 [internal] load git source https://github.com/tonistiigi/xx.git#refs/tags/v1.9.0
#1 0.625 a5592eab7a57895e8d385394ff12241bc65ecd50       refs/tags/v1.9.0
#1 0.051 Initialized empty Git repository in /var/lib/desktop-containerd/daemon/io.containerd.snapshotter.v1.overlayfs/snapshots/23414/fs/
#1 0.699 From https://github.com/tonistiigi/xx
#1 0.699  * [new tag]         v1.9.0     -> v1.9.0
#1 0.703 a5592eab7a57895e8d385394ff12241bc65ecd50
#1 DONE 1.4s
{
  "group": {
    "default": {
      "targets": [
        "xx"
      ]
    }
  },
  "target": {
    "xx": {
      "context": "https://github.com/tonistiigi/xx.git#refs/tags/v1.9.0:src",
      "dockerfile": "Dockerfile",
      "tags": [
        "tonistiigi/xx:test"
      ],
      "target": "xx"
    }
  }
}

docker buildx bake "https://github.com/tonistiigi/xx.git?tag=v1.9.0" xx --print
#0 building with "default" instance using docker driver

#1 [internal] load git source https://github.com/tonistiigi/xx.git?tag=v1.9.0
#1 0.493 a5592eab7a57895e8d385394ff12241bc65ecd50       refs/tags/v1.9.0
#1 CACHED
{
  "group": {
    "default": {
      "targets": [
        "xx"
      ]
    }
  },
  "target": {
    "xx": {
      "context": "https://github.com/tonistiigi/xx.git?tag=v1.9.0#:src",
      "dockerfile": "Dockerfile",
      "tags": [
        "tonistiigi/xx:test"
      ],
      "target": "xx"
    }
  }
}

$ docker buildx bake "https://github.com/crazy-max/buildx-buildkit-tests.git#:buildx-3682" --print
#0 building with "default" instance using docker driver

#1 [internal] load git source https://github.com/crazy-max/buildx-buildkit-tests.git#:buildx-3682
#1 0.501 ref: refs/heads/main   HEAD
#1 0.501 848d0e37436a9228dede5abd71f447d63ec380c9       HEAD
#1 0.881 848d0e37436a9228dede5abd71f447d63ec380c9       refs/heads/main
#1 0.051 Initialized empty Git repository in /var/lib/desktop-containerd/daemon/io.containerd.snapshotter.v1.overlayfs/snapshots/23416/fs/
#1 0.389 ref: refs/heads/main   HEAD
#1 0.389 848d0e37436a9228dede5abd71f447d63ec380c9       HEAD
#1 0.940 From https://github.com/crazy-max/buildx-buildkit-tests
#1 0.940  * [new branch]      main       -> main
#1 0.941  * [new branch]      main       -> origin/main
#1 0.944 848d0e37436a9228dede5abd71f447d63ec380c9
#1 DONE 2.0s
{
  "group": {
    "default": {
      "targets": [
        "default"
      ]
    }
  },
  "target": {
    "default": {
      "context": "https://github.com/crazy-max/buildx-buildkit-tests.git#:buildx-3682/src",
      "dockerfile": "Dockerfile"
    }
  }
}

$ docker buildx bake "https://github.com/crazy-max/buildx-buildkit-tests.git?subdir=buildx-3682" --print
#0 building with "default" instance using docker driver

#1 [internal] load git source https://github.com/crazy-max/buildx-buildkit-tests.git?subdir=buildx-3682
#1 0.391 ref: refs/heads/main   HEAD
#1 0.391 848d0e37436a9228dede5abd71f447d63ec380c9       HEAD
#1 0.768 848d0e37436a9228dede5abd71f447d63ec380c9       refs/heads/main
#1 CACHED
{
  "group": {
    "default": {
      "targets": [
        "default"
      ]
    }
  },
  "target": {
    "default": {
      "context": "https://github.com/crazy-max/buildx-buildkit-tests.git?subdir=buildx-3682%2Fsrc",
      "dockerfile": "Dockerfile"
    }
  }
}

[tonistiigi]

this should be subdir != "" ?