fsck --name-objects: be more careful parsing generation numbers
In 7b35efd734e (fsck_walk(): optionally name objects on the go, 2016-07-17), the `fsck` machinery learned to optionally name the objects, so that it is easier to see what part of the repository is in a bad shape, say, when objects are missing. To save on complexity, this machinery uses a parser to determine the name of a parent given a commit's name: any `~<n>` suffix is parsed and the parent's name is formed from the prefix together with `~<n+1>`. However, this parser has a bug: if it finds a suffix `<n>` that is _not_ `~<n>`, it will mistake the empty string for the prefix and `<n>` for the generation number. In other words, it will generate a name of the form `~<bogus-number>`. Let's fix this. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>