scripts: hide double git reference check error

scripts: hide double git reference check error

When checking a git reference which does not exist, a git error
with the long git-branch usage is printed:

% scripts/check-git-log.sh '-1 3780cbd'
error: malformed object name 2de9f8551ff9
usage: git branch ...
  [a lot of lines]
Wrong 'Fixes' reference:
  Fixes: 2de9f8551ff9 ("ethdev: fix documentation for queue start/stop")

The error from the script is sufficient so the git error can be hidden.

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>