Fix ambiguous dates in commit graph

What does this MR do and why?

This MR fixes an issue with ambiguous dates in the repository graph view.

If consecutive commit dates fall on different years but same month, the day numbers can be confusing, since the month label is not rendered.

This MR adds the missing month labels in these cases.

Refs #22315 (comment 603697697) which maybe should have been its own issue..?

I think this is a good step, if small, towards clarity.

The next step could be to add the year to be always visible, if that is still wanted.

Screenshots or screen recordings

How to set up and validate locally

1. Visit the repository graph (Network) of a project with consecutive commits in the same month but different year (help needed: anyone know of a public project that could be imported?).
2. See that the month labels are rendered, where year changes.

MR acceptance checklist

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