Handle repos with no commits

Repos with no commits caused an IllegalArgumentException when trying to
set the build metadata. This change models repos with no commits by
having the commit ID be an Optional and falls back to "uncommitted" if
no commit ID is found when settting the build metadata.

This fixes #68.

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

@@ -61,8 +61,8 @@ public VcsInventory(
             .orElse(Collections.emptySet());
   }
 
-  public String getCommitId() {
-    return commitId;
+  public Optional<String> getCommitId() {
+    return Optional.ofNullable(commitId);
   }
 
   public Optional<Version> getCurrentVersion() {

reckon-core/src/main/java/org/ajoberstar/reckon/core/strategy/StagePreReleaseStrategy.java

@@ -84,7 +84,7 @@ public Version reckonTargetVersion(VcsInventory inventory, Version targetNormal)
       return targetBase
           .setPreReleaseVersion(
               baseStageName + "." + baseStageNum + "." + inventory.getCommitsSinceBase())
-          .setBuildMetadata(inventory.getCommitId());
+          .setBuildMetadata(inventory.getCommitId().orElse("uncommitted"));
     } else if (stage.equals(baseStageName)) {
       int num = baseStageNum > 0 ? baseStageNum + 1 : 1;
       return targetBase.setPreReleaseVersion(stage + "." + num);

reckon-core/src/test/groovy/org/ajoberstar/reckon/core/strategy/StagePreReleaseStrategyTest.groovy

@@ -68,6 +68,22 @@ class StagePreReleaseStrategyTest extends Specification {
     strategy('rc').reckonTargetVersion(inventory, Version.valueOf('2.0.0')) == Version.valueOf('2.0.0-rc.1')
   }
 
+  def 'if repo has no commits, show build metadata as uncommitted'() {
+    given:
+    def inventoryUncommitted = new VcsInventory(
+      null,
+      null,
+      Version.valueOf('1.2.3-milestone.2'),
+      Version.valueOf('1.2.2'),
+      5,
+      [] as Set,
+      [] as Set
+    )
+    expect:
+    strategy(null).reckonTargetVersion(inventoryUncommitted, Version.valueOf('1.2.3')) == Version.valueOf('1.2.3-milestone.2.5+uncommitted')
+
+  }
+
   private StagePreReleaseStrategy strategy(String stage) {
     return new StagePreReleaseStrategy(['initial', 'milestone', 'rc', 'final'] as Set, { i, v -> Optional.ofNullable(stage) })
   }