newvers.sh: accept "git-svn-id:" at the start of a line only

This prevents incorrect subversion revision detection when "git svn" is
not being used to get the sources but git is available. Previously old
subversion revisions included in commit messages were favoured over the
more recent and correct revisions in git notes.

For example cf1f35574722 represents r315395 but was treated as r313908
which is referenced in the commit message. Commits following
r315395/cf1f35574722 but before another commit with a git-svn-id
reference in the commit message would be treated as r313908 as well.

Patch from PR updated to accommodate the initial four space indent in
git log ouptut.

PR: 221848
Submitted by: Fabian Keil
Obtained from: ElectroBSD
MFC after: 2 weeks