gittest: Allow writing arbitrary references in WriteCommit

The WriteCommit helper function has an option to write the newly written
commit to a specific branch. It's not possible though to write to an
arbitrary reference that is not a branch.

Add another option `WithReference()` that allows callers to chose custom
reference prefixes so that they can write non-branch-references.

2 files changed, 27 insertions, 7 deletions

diff --git a/internal/git/gittest/commit.go b/internal/git/gittest/commit.go
index 526e73545..3ffc1035a 100644
--- a/internal/git/gittest/commit.go
+++ b/internal/git/gittest/commit.go
@@ -32,7 +32,7 @@ var (
 )
 
 type writeCommitConfig struct {
-	branch             string
+	reference          string
 	parents            []git.ObjectID
 	authorDate         time.Time
 	authorName         string
@@ -47,14 +47,20 @@ type writeCommitConfig struct {
 // WriteCommitOption is an option which can be passed to WriteCommit.
 type WriteCommitOption func(*writeCommitConfig)
 
-// WithBranch is an option for WriteCommit which will cause it to update the update the given branch
-// name to the new commit.
-func WithBranch(branch string) WriteCommitOption {
+// WithReference is an option for WriteCommit which will cause it to update the given reference to
+// point to the new commit. This function requires the fully-qualified reference name.
+func WithReference(reference string) WriteCommitOption {
 	return func(cfg *writeCommitConfig) {
-		cfg.branch = branch
+		cfg.reference = reference
 	}
 }
 
+// WithBranch is an option for WriteCommit which will cause it to update the given branch name to
+// the new commit.
+func WithBranch(branch string) WriteCommitOption {
+	return WithReference("refs/heads/" + branch)
+}
+
 // WithMessage is an option for WriteCommit which will set the commit message.
 func WithMessage(message string) WriteCommitOption {
 	return func(cfg *writeCommitConfig) {
@@ -218,10 +224,10 @@ func WriteCommit(t testing.TB, cfg config.Cfg, repoPath string, opts ...WriteCom
 	oid, err := DefaultObjectHash.FromHex(text.ChompBytes(stdout))
 	require.NoError(t, err)
 
-	if writeCommitConfig.branch != "" {
+	if writeCommitConfig.reference != "" {
 		ExecOpts(t, cfg, ExecConfig{
 			Env: env,
-		}, "-C", repoPath, "update-ref", "refs/heads/"+writeCommitConfig.branch, oid.String())
+		}, "-C", repoPath, "update-ref", writeCommitConfig.reference, oid.String())
 	}
 
 	return oid
diff --git a/internal/git/gittest/commit_test.go b/internal/git/gittest/commit_test.go
index d1f279554..b4fa49a17 100644
--- a/internal/git/gittest/commit_test.go
+++ b/internal/git/gittest/commit_test.go
@@ -125,6 +125,20 @@ func TestWriteCommit(t *testing.T) {
 			expectedRevUpdate: "refs/heads/foo",
 		},
 		{
+			desc: "with reference",
+			opts: []WriteCommitOption{
+				WithReference("refs/custom/namespace"),
+			},
+			expectedCommit: strings.Join([]string{
+				"tree " + DefaultObjectHash.EmptyTreeOID.String(),
+				"author " + DefaultCommitterSignature,
+				"committer " + DefaultCommitterSignature,
+				"",
+				"message",
+			}, "\n"),
+			expectedRevUpdate: "refs/custom/namespace",
+		},
+		{
 			desc: "with tree entry",
 			opts: []WriteCommitOption{
 				WithTreeEntries(treeEntryA),