Update git-version for the vN.M release tagging scheme.

We need the sed invocation to strip the 'v' prefix
when naming the dist tarball.

doc/git-version.sh

@@ -2,7 +2,8 @@
 
 # script to build a version string
 
-GIT_VERSION=$(git describe --tags --dirty 2> /dev/null)
+GIT_VERSION=$(git describe --tags --match 'v*' --dirty 2> /dev/null)
+GIT_VERSION=$(echo ${GIT_VERSION} | sed 's/^v//')
 if test -z ${GIT_VERSION}; then
   VERSION='unknown'
 else