Make CommitList display "Working tree clean"

[jensenr30]

When there are no changes in the working directory, the top placeholder commit says "Working tree clean".

I added a function bool is_working_tree_clean(void) const in class CommitModel : public QAbstractListModel. Is there a better place to put this function? I'm a C++ beginner. I was trying to figure out how to access the is_working_tree_clean() function in other places in CommitList.cpp but couldn't figure out a clean way to do it...

Resolves #704

[Murmele]

Thanks for the changes. Is this ready? I think the is_working_tree_clean function is fine there

[jensenr30]

@Murmele I think this is good to merge in, but it might be nice to merge #714 first, because that fixes a bug with this element. But it's your call of course.

[jensenr30]

@Murmele I will leave this as a Draft until #714 gets merged in because this depends on #714.

[Murmele]

Now this can be rebased as well :)

[jensenr30]

Hmmm... Looks like I broke some tests... I'll look into this...