t6112: avoid tilde expansion

t6112: avoid tilde expansion

e987df5fe6 (list-objects-filter: implement composite filters,
2019-06-27) introduced a test to "t6112-rev-list-filters-objects.sh"
that checks the output of a Git command with the following commands:

  grep ~$omitted_1 actual &&
  grep ~$omitted_2 actual &&
  grep ~$omitted_3 actual &&

Since the leading tilde in the pattern is not quoted/escaped, it is
subject to tilde expansion.  So if the system has a user whose
username happens to be "$omitted_1", then "grep" would look for that
user's home directory.

Quote those words starting with a tilde to avoid this.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>