Use abbreviated hash in build metedata

Since the commit ID is only used for metadata on insignificant versions,
there's no reason to be so concerned about uniqueness that we use a full
hash. It will be easier for users to deal with if its shorter. Using the
ObjectReader method in this change, does ensure uniqueness within the
current repo, which is all we really need anyway.

This fixes #86.

reckon-core/src/main/java/org/ajoberstar/reckon/core/GitInventorySupplier.java

@@ -20,6 +20,7 @@
 import org.eclipse.jgit.api.errors.GitAPIException;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.revwalk.RevCommit;
@@ -55,7 +56,7 @@ public GitInventorySupplier(Repository repo, Function<String, Optional<String>>
   @Override
   public VcsInventory getInventory() {
     // share this walk throughout to benefit from its caching
-    try (RevWalk walk = new RevWalk(repo)) {
+    try (ObjectReader reader = repo.newObjectReader(); RevWalk walk = new RevWalk(reader)) {
       // saves on some performance as we don't really need the commit bodys
       walk.setRetainBody(false);
 
@@ -94,7 +95,7 @@ public VcsInventory getInventory() {
       Set<Version> claimedVersions = taggedVersions.stream().map(TaggedVersion::getVersion).collect(Collectors.toSet());
 
       return new VcsInventory(
-          headObjectId.getName(),
+          reader.abbreviate(headObjectId).name(),
           isClean(),
           currentVersion,
           baseVersion.getVersion(),

reckon-core/src/test/groovy/org/ajoberstar/reckon/core/GitInventorySupplierTest.groovy

@@ -15,6 +15,11 @@ class GitInventorySupplierTest extends Specification {
 
   GitInventorySupplier supplier
 
+  def 'commit id is abbreviated, not full'() {
+    expect:
+    supplier.getInventory().commitId == Optional.of(grgit.head().abbreviatedId)
+  }
+
   def 'if HEAD has no tagged versions, current version is empty'() {
     given:
     checkout('head-untagged')