Add support for resolving refs, when linked.

With ko-build/ko#73 we can link the Git ref information
into images with:
```
ln -r -s .git/refs ./cmd/foo/kodata
```

If the changeset package gets a `ref: <ref>` entry, it attempts to read that
file from `KO_DATA_PATH` next, which lets the changeset work much more effectively.

changeset/commit.go

@@ -42,11 +42,18 @@ func Get() (string, error) {
 		return "", err
 	}
 	commitID := strings.TrimSpace(string(data))
-	if !commitIDRE.MatchString(commitID) {
-		err := fmt.Errorf("%q is not a valid GitHub commit ID", commitID)
-		return "", err
+	if strings.HasPrefix(commitID, "ref: ") {
+		refName := commitID[len("ref: "):]
+		data, err := readFileFromKoData(refName)
+		if err != nil {
+			return "", err
+		}
+		commitID = strings.TrimSpace(string(data))
+	}
+	if commitIDRE.MatchString(commitID) {
+		return commitID[:7], nil
 	}
-	return string(commitID[0:7]), nil
+	return "", fmt.Errorf("%q is not a valid GitHub commit ID", commitID)
 }
 
 // readFileFromKoData tries to read data as string from the file with given name

changeset/commit_test.go

@@ -24,8 +24,8 @@ import (
 )
 
 const (
-	nonCommittedHeadContext = "ref: refs/heads/non_committed_branch"
-	testCommitID            = "a2d1bdf"
+	nonCommittedHeadRef = "refs/heads/non_committed_branch"
+	testCommitID        = "a2d1bdf"
 )
 
 func TestReadFile(t *testing.T) {
@@ -41,10 +41,19 @@ func TestReadFile(t *testing.T) {
 		koDataPath: "testdata",
 		want:       testCommitID,
 	}, {
-		name:       "non committed branch",
+		name:       "no refs link",
 		koDataPath: "testdata/noncommitted",
 		wantErr:    true,
-		err:        fmt.Errorf("%q is not a valid GitHub commit ID", nonCommittedHeadContext),
+		err:        fmt.Errorf("open testdata/noncommitted/%s: no such file or directory", nonCommittedHeadRef),
+	}, {
+		name:       "with refs link",
+		koDataPath: "testdata/with-refs",
+		want:       testCommitID,
+	}, {
+		name:       "with bad content",
+		koDataPath: "testdata/garbage",
+		wantErr:    true,
+		err:        fmt.Errorf(`"garbage contents" is not a valid GitHub commit ID`),
 	}, {
 		name:       "KO_DATA_PATH is empty",
 		koDataPath: "",

changeset/testdata/garbage/HEAD

@@ -0,0 +1 @@
+garbage contents

changeset/testdata/with-refs/HEAD

@@ -0,0 +1 @@
+ref: refs/heads/branch-name

changeset/testdata/with-refs/refs/heads/branch-name

@@ -0,0 +1 @@
+a2d1bdfe929516d7da141aef68631a7ee6941b2d