This PR removes the commit preview popover when hovering over rows in
the git graph. In my humble opinion, I've been finding that having the
hover preview damages the usability/readability of the graph. I can't
properly read each row's content and understand its place on the graph
if everywhere I rest my cursor in a popover appears... Moreover, almost
all of the information we display in the commit preview is already
displayed in the row itsself, and if you'd like to drill down into the
commit description and whatnot, clicking the row opens the commit drawer
and gives you that and even more information (e.g., set of changed
files, etc.).

Release Notes:

- N/A