[Bug other/102664] contrib/gcc-git-customization.sh uses echo -n, which isn't portable

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102664

--- Comment #21 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We should use: sed -n 's,^tags/,,;s,^basepoints/gcc-,r,p'