bug#15322: VC log buffer scrolls itself

[Dmitry Gutov]

Andreas Schwab <schwab@suse.de> writes:

> IMHO scrolling should only happen if point is still at BOB.

While this makes sense, and would help with fixing bug#20470 without
re-introducing the main pain point of this one, we don't have any easy
access to the "current" value of point, neither from
`vc-print-log-internal' (which is called before any output is shown),
nor from within the goto-location-func lambda it passes
`vc-log-internal-common', because the latter is called from
`vc--process-sentinel', after point is temporarily moved to
(process-mark p).