contrib: run fetch before pushing Daily bump
As seen from recent days, the script fails when it pushes
a branch while another revision was pushed by a user.
Prevent that by doing fetch right before the pull.
The error message example:
cmdline: git push origin releases/gcc-11
stderr: 'fatal: unable to parse object:
4249a65c814287af667aa78789436d3fc618e80a
error: remote unpack failed: eof before pack header was fully read
contrib/ChangeLog:
* gcc-changelog/git_update_version.py: Do fetch before a push.