maint.mk: restore functionality removed by recent change

maint.mk: restore functionality removed by recent change

Sunday's change, v0.0-8062-g6b24f60, may have appeared correct from
the context of a shallow-cloned gnulib repository: "git describe"
would fail in such a directory.  However, that change made it so
the reported gnulib revision no longer includes the version number
or a commit count, even when run from a full clone.
* top/maint.mk (gnulib-version): Use the full "git describe"
output when possible, e.g., the form above, rather than the
abbreviated, no-tag, no-commit-count string, and fall back to
using a 10-byte hash, rather than the default minimal-length
hash prefix, since while the minimal-length one may be fine today,
it is likely not to be unique for very long.