patch: Use reference only as an alternate obj store

Old behavior was to use the reference repo (usually the project's
checkout repo) as a full remote that we would fetch from. However, Git
has a more direct way to reference an existing repo, using
`objects/info/alternates`. This mechanism will just let you reuse
objects that already exist in the reference repo rather than fetching
from the real remotes.

So now only origin is a remote and we just try to reuse objects from
the remote. This also simplifies things where we had to check if the
branch we cared about existed. Now, we don't care and if the objects
are relevant they get used and if not, it should be no harm, no foul.

This will hopefully address the issue that reported in #112.

src/compatTest/groovy/org/ajoberstar/gradle/git/publish/BaseCompatTest.groovy

@@ -127,7 +127,7 @@ gitPublish {
     working.close()
   }
 
-  def 'reset pulls from reference repo if available before pulling from remote'() {
+  def 'reset uses reference repo objects if available before pulling from remote'() {
     given:
     def referenceDir = new File(tempDir, 'reference')
     def reference = Grgit.clone(dir: referenceDir, uri: repoPath(remote))
@@ -159,7 +159,6 @@ gitPublish {
     remote.checkout(branch: 'gh-pages')
     then:
     result.task(':gitPublishPush').outcome == TaskOutcome.SUCCESS
-    result.output.contains('gh-pages   -> reference/gh-pages')
     remote.log().size() == 2
     remoteFile('content.txt').text == 'published content here'
     !remoteFile('newFile.txt').exists()
@@ -328,7 +327,6 @@ gitPublish {
     }
 
     def working = Grgit.clone(dir: "${projectDir}/build/gitPublish", uri: badRemote.repository.rootDir.toURI())
-    working.checkout(branch: 'gh-pages', startPoint: 'origin/master', createBranch: true)
     working.close()
 
     new File(projectDir, 'content.txt') << 'published content here'
@@ -350,9 +348,11 @@ gitPublish {
     and:
     remote.checkout(branch: 'gh-pages')
     working = Grgit.open(dir: "${projectDir}/build/gitPublish")
+    working.checkout(branch: 'master')
     then:
     result.task(':gitPublishPush').outcome == TaskOutcome.SUCCESS
     remote.log().size() == 2
+    working.head().fullMessage == 'bad first commit'
   }
 
   def 'when no git publish tasks are run, build completes successfully'() {

src/main/java/org/ajoberstar/gradle/git/publish/tasks/GitPublishReset.java

@@ -4,7 +4,10 @@
 import java.io.IOException;
 import java.io.OutputStream;
 import java.io.UncheckedIOException;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
 
 import javax.inject.Inject;
 
@@ -91,51 +94,21 @@ public void reset() throws IOException {
       });
     }
 
-    // set reference
+    // set alternate object store if reference used
     if (getReferenceRepoUri().isPresent()) {
-      try {
-        getExecOperations().exec(spec -> {
-          spec.commandLine("git", "remote", "add", "reference", getReferenceRepoUri().get());
-          spec.workingDir(repoDir);
-          spec.setStandardOutput(OutputStream.nullOutputStream());
-          spec.setErrorOutput(OutputStream.nullOutputStream());
-        });
-      } catch (Exception e) {
-        getExecOperations().exec(spec -> {
-          spec.commandLine("git", "remote", "set-url", "reference", getReferenceRepoUri().get());
-          spec.workingDir(repoDir);
-          spec.setStandardOutput(OutputStream.nullOutputStream());
-        });
-      }
-
-      // check reference for branch
-      boolean referenceHasBranch;
-      try {
-        getExecOperations().exec(spec -> {
-          spec.commandLine("git", "ls-remote", "--exit-code", "reference", pubBranch);
-          spec.workingDir(repoDir);
-          spec.setStandardOutput(OutputStream.nullOutputStream());
-        });
-        referenceHasBranch = true;
-      } catch (Exception e) {
-        referenceHasBranch = false;
-      }
-
-      if (referenceHasBranch) {
-        // get local branch reset to remote state
-        getExecOperations().exec(spec -> {
-          var refSpec = String.format("+refs/heads/%s:refs/remotes/reference/%s", pubBranch, pubBranch);
-
-          spec.executable("git");
-          spec.args("fetch");
-          if (getFetchDepth().isPresent()) {
-            spec.args("--depth", getFetchDepth().get());
-          }
-          spec.args("reference", refSpec);
-
-          spec.workingDir(repoDir);
-          spec.setStandardOutput(OutputStream.nullOutputStream());
-        });
+      Path repoObjectsPath = repoDir.toPath().resolve(".git").resolve("objects");
+      Path alternatesPath = repoObjectsPath.resolve("info").resolve("alternates");
+
+      Path referenceRepoPath = Path.of(getReferenceRepoUri().get());
+
+      Path referenceRepoObjectsPath = referenceRepoPath.resolve(".git").resolve("objects");
+      Path referenceBareRepoObjectsPath = referenceRepoPath.resolve("objects");
+      if (Files.exists(referenceRepoObjectsPath)) {
+        Files.writeString(alternatesPath, referenceRepoObjectsPath + "\n", StandardCharsets.UTF_8);
+      } else if (Files.exists(referenceBareRepoObjectsPath)) {
+        Files.writeString(alternatesPath, referenceBareRepoObjectsPath + "\n", StandardCharsets.UTF_8);
+      } else {
+        getLogger().warn("Reference repo doesn't seem to have an objects database: {}", referenceRepoPath);
       }
     }
 
@@ -146,6 +119,7 @@ public void reset() throws IOException {
         spec.commandLine("git", "ls-remote", "--exit-code", "origin", pubBranch);
         spec.workingDir(repoDir);
         spec.setStandardOutput(OutputStream.nullOutputStream());
+        spec.setErrorOutput(OutputStream.nullOutputStream());
       });
       hasBranch = true;
     } catch (Exception e) {