contrib: Fix non-portable sed commands in gcc-descr [PR102664/]

POSIX sed does not support \? or \+ in its Basic Regular Expression
grammar. Replace the \(tags/\)\? part of the pattern with a substitution
to remove ^tags/ before other substitutions. Replace \([0-9]\+\) with
\([0-9][0-9]*\) or with \([1-9][0-9]*\) in release branch numbers, where
a leading zero does not occur.

contrib/ChangeLog:

	PR other/102664
	* git-descr.sh: Use portable sed commands.
	* git-undescr.sh: Likewise.