generate-git-snapshot: explicitely check for beginning of line when retrieving version information

If the changelog contains the string "Version" then when
identifying the original version number this can go wrong
obviously, e.g.:

| + version_information
| ++ dpkg-parsechangelog --count 1
| ++ awk '/Version/ {print $2}'
| + ORIG_VERSION='1.17.2
| Version'

Adjust the dpkg-parsechangelog command line to what's being
used in all the other command lines.