Break git.merging.args config into separate arguments on whitespace

This makes it possible again to pass multiple arguments, for example
"--ff-only --autostash". This won't work correctly if you want to use
an argument that contains a space, but it's very unlikely that people
will want to do that, so I think this is good enough.

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

@@ -138,8 +138,7 @@ func TestBranchMerge(t *testing.T) {
 			},
 			opts:       MergeOpts{},
 			branchName: "mybranch",
-			expected:   []string{"merge", "--no-edit", "--arg1 --arg2", "mybranch"},
-			// This is wrong, we want separate arguments for "--arg1" and "--arg2"
+			expected:   []string{"merge", "--no-edit", "--arg1", "--arg2", "mybranch"},
 		},
 		{
 			testName:   "fast forward only",