Don't return commits from setCommitMergedStatuses

Since the slice stores pointers to objects, and we're only modifying the objects
but not the slice itself, there's no need to return it and assign it back. This
will allow us to call the function for subslices of commits.

Also, move the condition that checks for an empty string inside the function;
we're going to call it from more than one place, so this makes it easier.

pkg/commands/git_commands/commit_loader_test.go

@@ -548,7 +548,8 @@ func TestCommitLoader_setCommitMergedStatuses(t *testing.T) {
 
 	for _, scenario := range scenarios {
 		t.Run(scenario.testName, func(t *testing.T) {
-			expectedCommits := setCommitMergedStatuses(scenario.ancestor, scenario.commits)
+			expectedCommits := scenario.commits
+			setCommitMergedStatuses(scenario.ancestor, expectedCommits)
 			assert.Equal(t, scenario.expectedCommits, expectedCommits)
 		})
 	}