version_git.sh: cope with wcs that emit whitespace.

Some versions of wc (notably the one supplied by OS X) prepend
whitespace to the output of `wc -l`, breaking the indentation of the
generated version_git.jl. We strip this using only tr(1) features
specified in
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/tr.html.