contrib/gcc-changelog: Skip over review lines

contrib/gcc-changelog: Skip over review lines

git-check-commit.py does not know about "Reviewed-by",
"Reviewed-on", and "Signed-off-by" lines and hence it
expects those lines which follow the ChangeLog entries
to be indented by a tab.

This commit makes the script skip those lines.  No further
processing is attempted because the review information
is not part of the ChangeLogs.

contrib/

2020-05-20  Frederik Harwath  <frederik@codesourcery.com>

	* gcc-changelog/git_commit.py: Skip over lines starting
	with "Reviewed-by: ", "Reviewed-on: ", or "Signed-off-by: "