Merge branch 'jh/empty-notes'

A request to store an empty note via "git notes" meant to remove
note from the object but with --allow-empty we will store a (surprise!)
note that is empty.  In the longer run, we might want to deprecate
the somewhat unintuitive "emptying means deletion" behaviour.

* jh/empty-notes:
  t3301: modernize style
  notes: empty notes should be shown by 'git log'
  builtin/notes: add --allow-empty, to allow storing empty notes
  builtin/notes: split create_note() to clarify add vs. remove logic
  builtin/notes: simplify early exit code in add()
  builtin/notes: refactor note file path into struct note_data
  builtin/notes: improve naming
  t3301: verify that 'git notes' removes empty notes by default
  builtin/notes: fix premature failure when trying to add the empty blob

Documentation/git-notes.txt

@@ -9,10 +9,10 @@ SYNOPSIS
 --------
 [verse]
 'git notes' [list [<object>]]
-'git notes' add [-f] [-F <file> | -m <msg> | (-c | -C) <object>] [<object>]
+'git notes' add [-f] [--allow-empty] [-F <file> | -m <msg> | (-c | -C) <object>] [<object>]
 'git notes' copy [-f] ( --stdin | <from-object> <to-object> )
-'git notes' append [-F <file> | -m <msg> | (-c | -C) <object>] [<object>]
-'git notes' edit [<object>]
+'git notes' append [--allow-empty] [-F <file> | -m <msg> | (-c | -C) <object>] [<object>]
+'git notes' edit [--allow-empty] [<object>]
 'git notes' show [<object>]
 'git notes' merge [-v | -q] [-s <strategy> ] <notes-ref>
 'git notes' merge --commit [-v | -q]
@@ -155,6 +155,10 @@ OPTIONS
 	Like '-C', but with '-c' the editor is invoked, so that
 	the user can further edit the note message.
 
+--allow-empty::
+	Allow an empty note object to be stored. The default behavior is
+	to automatically remove empty notes.
+
 --ref <ref>::
 	Manipulate the notes tree in <ref>.  This overrides
 	'GIT_NOTES_REF' and the "core.notesRef" configuration.  The ref
@@ -287,7 +291,7 @@ arbitrary files using 'git hash-object':
 ------------
 $ cc *.c
 $ blob=$(git hash-object -w a.out)
-$ git notes --ref=built add -C "$blob" HEAD
+$ git notes --ref=built add --allow-empty -C "$blob" HEAD
 ------------
 
 (You cannot simply use `git notes --ref=built add -F a.out HEAD`