cmd/cueckoo: add short commit hash to "Closes" for importpr

When we import a PR from GitHub we immediately lose track of the
original commit sha because of necessary commit message rewriting.
Currently this means we have no easy way of anyone tracking exactly what
was imported.

Per a suggestion from @mvdan, we can simply add this to the "Closes"
message for now, and later consider writing a message back to the PR
from which we are importing with more information.

For #33.

cmd/cueckoo/cmd/importpr.go

@@ -110,6 +110,12 @@ func importPRDef(c *Command, args []string) error {
 	}
 	log.Printf("fetched PR into branch %q", branchName)
 
+	// Extract the commit hash
+	commitHash, err := run(ctx, "git", "rev-parse", "--short", "HEAD")
+	if err != nil {
+		return fmt.Errorf("failed to establish commit hash: %w", err)
+	}
+
 	// Set the branch upstream as the first step. If subsequent commands fail
 	// (they shouldn't but it can happen) we still need the upstream to have
 	// been set.
@@ -160,7 +166,7 @@ func importPRDef(c *Command, args []string) error {
 	if err != nil {
 		return err
 	}
-	msg, err = addClosesMsg(msg, prNumber)
+	msg, err = addClosesMsg(msg, prNumber, commitHash)
 	if err != nil {
 		return err
 	}
@@ -218,7 +224,7 @@ func run(ctx context.Context, name string, args ...string) (string, error) {
 // git-interpret-trailers". If there are trailers we want to insert "Closes
 // #PR as merged." as the last clear line before the trailers. If there are
 // no trailers, it should be the final line in the commit message.
-func addClosesMsg(msg string, pr int) (string, error) {
+func addClosesMsg(msg string, pr int, commitHash string) (string, error) {
 	// TODO: handle carriage returns?
 
 	// Drop any trailing space. We will add back a \n at the end
@@ -240,7 +246,7 @@ func addClosesMsg(msg string, pr int) (string, error) {
 	msg = strings.TrimRightFunc(msg, unicode.IsSpace)
 
 	// Prepare the closes message
-	closes := fmt.Sprintf("Closes #%d as merged.", pr)
+	closes := fmt.Sprintf("Closes #%d as merged as of commit %v.", pr, commitHash)
 
 	// Add the closes message
 	msg += "\n\n" + closes

cmd/cueckoo/cmd/importpr_test.go

@@ -20,7 +20,7 @@ func TestAddCloses(t *testing.T) {
 My commit message with no trailer
 
 `,
-			out: "first line\n\nMy commit message with no trailer\n\nCloses #0 as merged.\n",
+			out: "first line\n\nMy commit message with no trailer\n\nCloses #0 as merged as of commit a01b2c3d.\n",
 		},
 		{
 			name: "signed-off-by trailer",
@@ -31,13 +31,13 @@ My commit message with no trailer
 Signed-off-by: Paul
 
 `,
-			out: "first line\n\nMy commit message with no trailer\n\nCloses #0 as merged.\n\nSigned-off-by: Paul\n",
+			out: "first line\n\nMy commit message with no trailer\n\nCloses #0 as merged as of commit a01b2c3d.\n\nSigned-off-by: Paul\n",
 		},
 	}
 
 	for _, c := range cases {
 		t.Run(c.name, func(t *testing.T) {
-			got, err := addClosesMsg(c.in, c.pr)
+			got, err := addClosesMsg(c.in, c.pr, "a01b2c3d")
 			if err != nil {
 				t.Fatalf("got error when none expected: %v", err)
 			}