t7501-commit: drop silly command substitution

t7501-commit: drop silly command substitution

The test '--dry-run with conflicts fixed from a merge' in
't7501-commit.sh', added in 8dc874b2ee (wt-status.c: set commitable
bit if there is a meaningful merge., 2016-02-15), runs the following
unnecessary and downright bogus command substitution:

  ! $(git merge --no-commit commit-1) &&

I.e. after 'git merge ...' is executed and expectedly fails, the test
attempts to execute its output:

  Merging:
  80f2ea2 commit 2
  virtual commit-1
  found 1 common ancestor:
  e60d113 Initial commit
  Auto-merging test-file
  CONFLICT (content): Merge conflict in test-file
  Automatic merge failed; fix conflicts and then commit the result.

as a command, which most likely fails, because there is no such
command as "Merging:".  Then '!' negates the failed exit status, the
test continues, and eventually succeeds.

Remove this command substitution and use 'test_must_fail' to ensure
that 'git merge' fails.

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