Add convenience builder for git commands

Adds a convenience builder for git commands. Applies it in a couple of places that benefit from it.

I've got a larger PR that does a more sweeping change but I want some reviews on that before I go ahead with it.

pkg/commands/git_commands/flow_test.go

@@ -0,0 +1,92 @@
+package git_commands
+
+import (
+	"testing"
+
+	"github.com/jesseduffield/lazygit/pkg/commands/git_config"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestStartCmdObj(t *testing.T) {
+	scenarios := []struct {
+		testName   string
+		branchType string
+		name       string
+		expected   string
+	}{
+		{
+			testName:   "basic",
+			branchType: "feature",
+			name:       "test",
+			expected:   "git flow feature start test",
+		},
+	}
+
+	for _, s := range scenarios {
+		s := s
+		t.Run(s.testName, func(t *testing.T) {
+			instance := buildFlowCommands(commonDeps{})
+
+			assert.Equal(t,
+				instance.StartCmdObj(s.branchType, s.name).ToString(),
+				s.expected,
+			)
+		})
+	}
+}
+
+func TestFinishCmdObj(t *testing.T) {
+	scenarios := []struct {
+		testName               string
+		branchName             string
+		expected               string
+		expectedError          string
+		gitConfigMockResponses map[string]string
+	}{
+		{
+			testName:               "not a git flow branch",
+			branchName:             "mybranch",
+			expected:               "",
+			expectedError:          "This does not seem to be a git flow branch",
+			gitConfigMockResponses: nil,
+		},
+		{
+			testName:               "feature branch without config",
+			branchName:             "feature/mybranch",
+			expected:               "",
+			expectedError:          "This does not seem to be a git flow branch",
+			gitConfigMockResponses: nil,
+		},
+		{
+			testName:      "feature branch with config",
+			branchName:    "feature/mybranch",
+			expected:      "git flow feature finish mybranch",
+			expectedError: "",
+			gitConfigMockResponses: map[string]string{
+				"--local --get-regexp gitflow.prefix": "gitflow.prefix.feature feature/",
+			},
+		},
+	}
+
+	for _, s := range scenarios {
+		s := s
+		t.Run(s.testName, func(t *testing.T) {
+			instance := buildFlowCommands(commonDeps{
+				gitConfig: git_config.NewFakeGitConfig(s.gitConfigMockResponses),
+			})
+
+			cmd, err := instance.FinishCmdObj(s.branchName)
+
+			if s.expectedError != "" {
+				if err == nil {
+					t.Errorf("Expected error, got nil")
+				} else {
+					assert.Equal(t, err.Error(), s.expectedError)
+				}
+			} else {
+				assert.NoError(t, err)
+				assert.Equal(t, cmd.ToString(), s.expected)
+			}
+		})
+	}
+}