List user branches on git web

[Frank Ch. Eigler]

Hi -

> > https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;a=heads;h=refs/users/marxin/heads
> 
> Apparently such page does not list user branches.

I believe this is because of unusual gcc git conventions, where the
refs/heads/ and refs/users/heads/ directories are distinct.  I believe
have gitweb expose those via the config knob extra-branch-refs, which
can also be set via .git/config.

> So a redirection from
> https://gcc.gnu.org/git-branches/marxin/heads/slp-function-v4

... if the above is done, then this probably is not necessary.

- FChE