Make it easy to create "amend!" commits

To support this, we turn the confirmation prompt of the "Create fixup commit"
command into a menu; creating a fixup commit is the first entry, so that
"shift-F, enter" behaves the same as before. But there are additional entries
for creating "amend!" commits, either with or without file changes. These make
it easy to reword commit messages of existing commits.

pkg/commands/git_commands/commit_test.go

@@ -180,6 +180,57 @@ func TestCommitCreateFixupCommit(t *testing.T) {
 	}
 }
 
+func TestCommitCreateAmendCommit(t *testing.T) {
+	type scenario struct {
+		testName           string
+		originalSubject    string
+		newSubject         string
+		newDescription     string
+		includeFileChanges bool
+		runner             *oscommands.FakeCmdObjRunner
+	}
+
+	scenarios := []scenario{
+		{
+			testName:           "subject only",
+			originalSubject:    "original subject",
+			newSubject:         "new subject",
+			newDescription:     "",
+			includeFileChanges: true,
+			runner: oscommands.NewFakeRunner(t).
+				ExpectGitArgs([]string{"commit", "-m", "amend! original subject", "-m", "new subject"}, "", nil),
+		},
+		{
+			testName:           "subject and description",
+			originalSubject:    "original subject",
+			newSubject:         "new subject",
+			newDescription:     "new description",
+			includeFileChanges: true,
+			runner: oscommands.NewFakeRunner(t).
+				ExpectGitArgs([]string{"commit", "-m", "amend! original subject", "-m", "new subject\n\nnew description"}, "", nil),
+		},
+		{
+			testName:           "without file changes",
+			originalSubject:    "original subject",
+			newSubject:         "new subject",
+			newDescription:     "",
+			includeFileChanges: false,
+			runner: oscommands.NewFakeRunner(t).
+				ExpectGitArgs([]string{"commit", "-m", "amend! original subject", "-m", "new subject", "--only", "--allow-empty"}, "", nil),
+		},
+	}
+
+	for _, s := range scenarios {
+		s := s
+		t.Run(s.testName, func(t *testing.T) {
+			instance := buildCommitCommands(commonDeps{runner: s.runner})
+			err := instance.CreateAmendCommit(s.originalSubject, s.newSubject, s.newDescription, s.includeFileChanges)
+			assert.NoError(t, err)
+			s.runner.CheckForMissingCalls()
+		})
+	}
+}
+
 func TestCommitShowCmdObj(t *testing.T) {
 	type scenario struct {
 		testName         string