notes.h/c: Allow combine_notes functions to remove notes
Allow combine_notes functions to request that a note be removed, by setting the resulting note SHA1 to null_sha1 (0000000...). For consistency, also teach note_tree_insert() to skip insertion of an empty note (a note with entry->val_sha1 equal to null_sha1) when there is no note to combine it with. In general, an empty note (null_sha1) is treated identically to no note at all, but when adding an empty note where there already exists a non-empty note, we allow the combine_notes function to potentially record a new/changed note. Document this behaviour, and clearly specify how combine_notes functions are expected to handle null_sha1 in input. Before this patch, storing null_sha1s in the notes tree were silently allowed, causing an invalid notes tree (referring to blobs with null_sha1) to be produced by write_notes_tree(). Signed-off-by: NJohan Herland <johan@herland.net> Signed-off-by: NJunio C Hamano <gitster@pobox.com>
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