When using URL to a git repo as build context allow specifying `commit`
to make sure specific `commit` is used as the build context.
Examples
```console
buildah build -t test myrepo.git#abcef123
buildah build -t test myrepo.git#abcef123
buildah build -t test myrepo.git#abcef123:somesubdir
```
Closes: https://github.com/containers/buildah/issues/4148
Signed-off-by: Aditya R <arajan@redhat.com>
Aditya R