Limit git history searches in newvers.sh

newvers.sh takes upwards of 4-5 seconds to complete on trees checked
out from github, due to searching the entire history for non-existent
git-svn metadata. Similarly, if one does not check out notes, we
again search the entire history for notes. That makes newvers.sh very
slow for many github users.

To fix this in a fair way, limit the history search to the last 10K
commits: if you're more than 10K commits out of sync, then you've
forked the project, and our SVN rev is no longer very important to you.

Due to how git implements --grep in conjunction with -n, --grep has been
removed for performance reasons (git does not seem to limit its search
to the -n limit in this case, and takes just as long as it did with no
limit).

Reviewed by: emaste, imp
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D18745