Fix: Git refs must enable pre-release selection

Considers a pre-release version at a Git refs is requesting a
pre-release version (i.e. opt-in to pre-releases), otherwise we skip
valid solutions.

Author: ysbaddaden

src/solver.cr

@@ -54,10 +54,11 @@ module Shards
 
       # pre-releases are opt-in, so we must check that the solution didn't
       # select one unless at least one requirement in the selected graph asked
-      # for it:
+      # for it, or we install the dependency at a Git refs:
       unless @prereleases
         packages.each do |package|
           next unless Versions.prerelease?(package.version)
+          next if package.commit
 
           if dependency = @spec.dependencies.find { |d| d.name == package.name }
             break if Versions.prerelease?(dependency.version)

test/integration/install_test.cr

@@ -71,6 +71,18 @@ class InstallCommandTest < Minitest::Test
     end
   end
 
+  def test_installs_prerelease_version_at_refs
+    metadata = {
+      dependencies: {
+        unstable: {git: git_url(:unstable), branch: "master"}
+      }
+    }
+    with_shard(metadata) do
+      run "shards install"
+      assert_installed "unstable", "0.3.0.beta"
+    end
+  end
+
   def test_installs_dependencies_at_locked_version
     metadata = {
       dependencies:             {web: "1.0.0"},