contrib: Verify the id to be printed is ancestor of the corresponding remote release...

contrib: Verify the id to be printed is ancestor of the corresponding remote release branch (or master), otherwise print nothing.

The monotonically increasing revision ids need to be globally unique, so they should
only identify commits that were committed to the upstream repo to its master or
releases/gcc-N branches.  The alias could print something even for private branches
or vendor branches etc., but if such an identifier is then used publicly, it will
refer to something else.

2020-01-16  Jakub Jelinek  <jakub@redhat.com>

	* gcc-git-customization.sh: Verify the id to be printed is ancestor of
	the corresponding remote release branch (or master), otherwise print
	nothing.