builtin/notes: Simplify early exit code in add()

Remove the need for 'retval' and the unnecessary goto. Also reorganize
to only call free_note_data() is actually needed.

Improved-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johan Herland <johan@herland.net>
builtin/notes: Split create_note() to clarify add vs. remove logic

create_note() has a non-trivial interface, and comprises three loosely
related parts:

 1. launching the editor with the note contents, if needed
 2. appending to an existing note, if append_only was given
 3. adding or removing the resulting note, based on whether it's non-empty

Split it along those lines to make the logic clearer: The first part
goes into a new function - prepare_note_data(), with a simpler interface.
The second part is moved into append_edit(), which is the only user of
this code. Finally, the add vs. remove decision is moved into the callers
(add() and append_edit()), keeping the logic for writing the actual note
object in a separate function: write_note_data().

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johan Herland <johan@herland.net>
builtin/notes: Add --allow-empty, to allow storing empty notes

Although the "git notes" man page advertises that we support binary-safe
notes addition (using the -C option), we currently do not support adding
the empty note (i.e. using the empty blob to annotate an object). Instead,
an empty note is always treated as an intent to remove the note
altogether.

Introduce the --allow-empty option to the add/append/edit subcommands,
to explicitly allow an empty note to be stored into the notes tree.

Also update the documentation, and add test cases for the new option.

Reported-by: James H. Fisher <jhf@trifork.com>
Improved-by: Kyle J. McKay <mackyle@gmail.com>
Improved-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johan Herland <johan@herland.net>
notes: Empty notes should be shown by 'git log'

If the user has gone through the trouble of explicitly adding an empty
note, then "git log" should not silently skip it (as if it didn't exist).

Signed-off-by: Johan Herland <johan@herland.net>
t3301: Modernize

Make this test script appear somewhat less old-fashioned:
 - Use test helper functions:
    - write_script
    - test_commit
    - test_write_lines
    - test_config
    - test_unconfig
    - test_path_is_missing
 - Remove whitespace between redirection operators and their targets.
 - Move preparation of "except" files into tests.
 - More consistent quoting, especially around commands that might
   expand to nothing.
 - More visibility of important whitespace with ${indent}.
 - Combine pairs of tests that unnecessarily split setup and verification.

Improved-by: Eric Sunshine <sunshine@sunshineco.com>
Improved-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johan Herland <johan@herland.net>