t: add tests for error conditions with --pathspec-from-file

t: add tests for error conditions with --pathspec-from-file

Also move some old tests into the new tests: it doesn't seem reasonable
to have individual error condition tests.

Old test for `git commit` was corrected, previously it was instructed
to use stdin but wasn't provided with any stdin. While this works at
the moment, it's not exactly perfect.

Old tests for `git reset` were improved to test for a specific error
message.

Suggested-By: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Alexandr Miloslavskiy <alexandr.miloslavskiy@syntevo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>