The usage of faces for filenames is inconsistent.

Hello again, Old One.

The usage of faces for filenames is inconsistent.

For one of my projects, the filenames look like this:

https://www.dropbox.com/s/tayezuw0vlij4uz/Screen%20Shot%202019-01-10%20at%2011.41.52%20PM.png?dl=0

They are fontified using `magit-section-heading`, which makes the filenames look like section headings, which I don't really want. I would prefer for them to instead use their own face and to inherit from e.g. `magit-section-heading` (although I would prefer a different face). This way, I can customize the color of the filenames without changing the look of the rest of magit.

For another project, the filenames look like this:

https://www.dropbox.com/s/81seb4ajcygdxxq/Screen%20Shot%202019-01-10%20at%2011.44.59%20PM.png?dl=0

It's a different color! Upon closer inspection, I discovered that `magit-filename` is used here.

So, two things:
1. The inconsistency is kind of confusing as the display of `magit-todos` changes depending on project. I'm guessing it depends on whether there is one file or multiple.
2. Magit faces are re-used, instead of defining new ones that simply inherit from existing faces.

I suggest fixing number (2) by introducing two new faces. Then someone can change them both to be the same color if they find two different colors to be confusing/inconsistent.

Speak your thoughts, Old One.