bug#32253: Creation of unchanged profiles

[Andreas Enge]

Something I have noticed just now with the new "guix pull", but which
also happens with regular profiles through "guix package": Sometimes
(always?) unchanged profiles are created with a new number.

For instance, here is the result of three "guix pull" in my profile:
$ guix pull -l
Generation 1    Jul 22 2018 23:47:42
  guix 9f2adb2
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: origin/master
    commit: 9f2adb2f19db6d2e2234df15b0e24c5d01b9181c
Generation 2    Jul 23 2018 14:43:30
  guix fef7bab
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: origin/master
    commit: fef7baba786a96b7a3100c9c7adf8b45782ced37
  1 new package: guile-pfds
  6 packages upgraded: address@hidden, address@hidden, address@hidden, 
address@hidden, address@hidden,
    address@hidden
Generation 3    Jul 23 2018 14:59:55    (aktuell)
  guix fef7bab
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: origin/master
    commit: fef7baba786a96b7a3100c9c7adf8b45782ced37

The third generation is identical to the second one and should not have
been created.

Andreas