notes.c: cleanup 'strbuf_grow' call in 'append_edit'

notes.c: cleanup 'strbuf_grow' call in 'append_edit'

Let's cleanup the unnecessary 'strbuf_grow' call in 'append_edit'. This
"strbuf_grow(&d.buf, size + 1);" is prepared for insert a blank line if
needed, but actually when inserting, "strbuf_insertstr(&d.buf, 0,
"\n");" will do the "grow" for us.

348f199b (builtin-notes: Refactor handling of -F option to allow
combining -m and -F, 2010-02-13) added these to mimic the code
introduced by 2347fae5 (builtin-notes: Add "append" subcommand for
appending to note objects, 2010-02-13) that reads in previous note
before the message.  And the resulting code with explicit sizing is
carried to this day.

In the context of reading an existing note in, exact sizing may have
made sense, but because the resulting note needs cleansing with
stripspace() when appending with this option, such an exact sizing
does not buy us all that much in practice.

It may help avoiding overallocation due to ALLOC_GROW() slop, but
nobody can feed so many long messages for it to matter from the
command line.

Signed-off-by: Teng Long <dyroneteng@gmail.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>