[PATCH 0/4] NO_PERL support

OK, here is a series based on Robin's patch that I think is suitable for inclusion in mainstream git. The first two are related cleanups, the third is a rebase of what I sent earlier today, and the fourth covers the matching tests.

  1/4: commit: abort commit if interactive add failed
  2/4: tests: remove exit after test_done call
  3/4: Makefile: allow building without perl
  4/4: tests: skip perl tests if NO_PERL is defined

With these applied, you can build and pass the tests with NO_PERL defined. _But_ you still can't pass the tests without perl installed at all, as several of the other tests rely on perl to do text munging in the tests. I'm not sure it is possible to rewrite them not to use perl; we ended up with perl in the first place because many versions of standard tools like sed don't handle NULs very well. So I think the only option would be to skip those tests, too.

Breaking my perl installation yields these failing tests:

  $ cat <<'EOF' >$HOME/local/bin
  #!/bin/sh
  echo >&2 I am a broken perl.
  exit 1
  EOF
  $ make test NO_PERL=NoThanks
  ...
  $ cd t && grep 'failed [^0]' test-results/*
  test-results/t0020-crlf-25091:failed 16
  test-results/t1300-repo-config-30265:failed 2
  test-results/t3300-funny-names-4931:failed 3
  test-results/t4012-diff-binary-22876:failed 1
  test-results/t4014-format-patch-23170:failed 14
  test-results/t4020-diff-external-25409:failed 2
  test-results/t4029-diff-trailing-space-26781:failed 1
  test-results/t4030-diff-textconv-26899:failed 5
  test-results/t4031-diff-rewrite-binary-26911:failed 1
  test-results/t4103-apply-binary-28344:failed 8
  test-results/t4116-apply-reverse-29276:failed 7
  test-results/t4200-rerere-30898:failed 1
  test-results/t5300-pack-object-983:failed 24
  test-results/t5303-pack-corruption-resilience-2980:failed 3
  test-results/t6002-rev-list-bisect-17175:failed 12
  test-results/t6003-rev-list-topo-order-17683:failed 31
  test-results/t6011-rev-list-with-bad-commit-19971:failed 3
  test-results/t6013-rev-list-reverse-parents-20684:failed 2
  test-results/t8001-annotate-11125:failed 10
  test-results/t8002-blame-11523:failed 10