t1304: Set LOGNAME even if USER is unset or null

t1304: Set LOGNAME even if USER is unset or null

Avoid:

  # ./t1304-default-acl.sh
  ok 1 - checking for a working acl setup
  ok 2 - Setup test repo
  not ok 3 - Objects creation does not break ACLs with restrictive umask
  #
  #               # SHA1 for empty blob
  #               check_perms_and_acl .git/objects/e6/9de29bb2d1d6434b8b29ae775ad8c2e48c5391
  #
  not ok 4 - git gc does not break ACLs with restrictive umask
  #
  #               git gc &&
  #               check_perms_and_acl .git/objects/pack/*.pack
  #
  # failed 2 among 4 test(s)
  1..4

on systems where USER isn't set.  It's usually set by the login
process, but it isn't set when launching some Docker images.  For
example:

  $ docker run --rm debian env
  HOME=/
  PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
  HOSTNAME=b2dfdfe797ed

'id -u -n' has been in POSIX from Issue 2 through 2013 [1], so I don't
expect compatibility issues.

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/id.html

Signed-off-by: W. Trevor King <wking@tremily.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>