Allow more than one argument in git.merging.args config (#3336)

- **PR Description**

Fix a bug where merging would fail with an error message when you try to
put more than one argument in the `git.merging.args` config. This broke
with 25f8b0337e.

Fixes #3334.

pkg/commands/git_commands/branch.go

@@ -210,7 +210,7 @@ type MergeOpts struct {
 func (self *BranchCommands) Merge(branchName string, opts MergeOpts) error {
 	cmdArgs := NewGitCmd("merge").
 		Arg("--no-edit").
-		ArgIf(self.UserConfig.Git.Merging.Args != "", self.UserConfig.Git.Merging.Args).
+		Arg(strings.Fields(self.UserConfig.Git.Merging.Args)...).
 		ArgIf(opts.FastForwardOnly, "--ff-only").
 		Arg(branchName).
 		ToArgv()

pkg/commands/git_commands/branch_test.go

@@ -127,6 +127,19 @@ func TestBranchMerge(t *testing.T) {
 			branchName: "mybranch",
 			expected:   []string{"merge", "--no-edit", "--merging-args", "mybranch"},
 		},
+		{
+			testName: "multiple merging args",
+			userConfig: &config.UserConfig{
+				Git: config.GitConfig{
+					Merging: config.MergingConfig{
+						Args: "--arg1 --arg2", // it's up to the user what they put here
+					},
+				},
+			},
+			opts:       MergeOpts{},
+			branchName: "mybranch",
+			expected:   []string{"merge", "--no-edit", "--arg1", "--arg2", "mybranch"},
+		},
 		{
 			testName:   "fast forward only",
 			userConfig: &config.UserConfig{},