range-diff: fix a crash in parsing git-log output

`git range-diff` calls `git log` internally and tries to parse its
output. But `git log` output can be customized by the user in their
git config and for certain configurations either an error will be
returned by `git range-diff` or it will crash.

To fix this explicitly set the output format of the internally
executed `git log` with `--pretty=medium`. Because that cancels
`--notes`, add explicitly `--notes` at the end.

Also, make sure we never crash in the same way - trying to dereference
`util` which was never created and has remained NULL. It would happen
if the first line of `git log` output does not begin with 'commit '.

Alternative considered but discarded - somehow disable all git configs
and behave as if no config is present in the internally executed
`git log`, but that does not seem to be possible. GIT_CONFIG_NOSYSTEM
is the closest to it, but even with that we would still read
`.git/config`.

Signed-off-by: Vasil Dimov <vd@FreeBSD.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
