gitk: use <Button-3> for ctx menus on macOS with Tcl 8.7+

gitk: use <Button-3> for ctx menus on macOS with Tcl 8.7+

Commit d277e89f87fda01daa1e1a35fc1f7118678faa1f added special handling
on macOS (OS X) that makes button 2 the right mouse button. As per TIP
474 [1], Tcl 8.7 has swapped buttons 2 and 3 such that button 3 is made
the right mouse button as in other platforms. Therefore, the logic
should be updated to use button 3 on macOS with Tcl 8.7+.

[1]: https://core.tcl-lang.org/tips/doc/main/tip/474.md

Signed-off-by: Ruoyu Zhong <zhongruoyu@outlook.com>