Hello,

When repacking a source checkout (e.g. by using git-fetch with a
snippet), the generated file name contains only the first two version
identifiers.

E.g. `guix build -S eudev` returns

/gnu/store/7lgsxmr0rk9f8fbq6k0kj1aqb7lnrlll-eudev-3.2.tar.xz

...even though it should be "3.2.8".

This patch fixes it: