vc-git-print-log: set buffer mode earlier

[Karl Chen]

I like to use `vc-print-log' then press 'd' on a log entry for
`log-view-diff'.

`vc-print-log' on a buffer in a git repository often returns the
first few log entries quickly then takes a while to finish
(especially if the disk cache is cold).

However, the log buffer isn't set as `vc-git-log-view-mode' until
the `git log' command completes.  Is it possible to do this
before `git log' completes?