Store Common instead of just the list of configured main branches in MainBranches

This will make it possible to change the configured main branches at runtime.
We'll support this in the next commit.

pkg/commands/git_commands/commit_loader_test.go

@@ -324,7 +324,7 @@ func TestGetCommits(t *testing.T) {
 
 			common.UserConfig().Git.MainBranches = scenario.mainBranches
 			opts := scenario.opts
-			opts.MainBranches = NewMainBranches(scenario.mainBranches, cmd)
+			opts.MainBranches = NewMainBranches(common, cmd)
 			commits, err := builder.GetCommits(opts)
 
 			assert.Equal(t, scenario.expectedCommits, commits)