Enable revive linter, and fix a bunch of warnings

I took the set of enabled checks from revive's recommended configuration [1],
and removed some that I didn't like. There might be other useful checks in
revive that we might want to enable, but this is a nice improvement already.

The bulk of the changes here are removing unnecessary else statements after
returns, but there are a few others too.

[1] https://github.com/mgechev/revive?tab=readme-ov-file#recommended-configuration

pkg/gui/presentation/graph/cell.go

@@ -177,7 +177,7 @@ func getBoxDrawingChars(up, down, left, right bool) (string, string) {
 		return "╶", "─"
 	} else if !up && !down && !left && !right {
 		return " ", " "
-	} else {
-		panic("should not be possible")
 	}
+
+	panic("should not be possible")
 }

pkg/gui/presentation/graph/graph.go

@@ -246,9 +246,8 @@ func getNextPipes(prevPipes []Pipe, commit *models.Commit, getStyle func(c *mode
 			for i := pipe.toPos; i > pos; i-- {
 				if takenSpots.Includes(int(i)) || traversedSpots.Includes(int(i)) {
 					break
-				} else {
-					last = i
 				}
+				last = i
 			}
 			newPipes = append(newPipes, Pipe{
 				fromPos:  pipe.toPos,