fsck: Support demoting errors to warnings

We already have support in `git receive-pack` to deal with some legacy
repositories which have non-fatal issues.

Let's make `git fsck` itself useful with such repositories, too, by
allowing users to ignore known issues, or at least demote those issues
to mere warnings.

Example: `git -c fsck.missingEmail=ignore fsck` would hide problems with
missing emails in author, committer and tagger lines.

In the same spirit that `git receive-pack`'s usage of the fsck machinery
differs from `git fsck`'s – some of the non-fatal warnings in `git fsck`
are fatal with `git receive-pack` when receive.fsckObjects = true, for
example – we strictly separate the fsck.* from the receive.fsck.*
settings.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: Introduce `git fsck --quick`

This option avoids unpacking each and all objects, and just verifies the
connectivity. In particular with large repositories, this speeds up the
operation, at the expense of missing corrupt blobs and ignoring
unreachable objects, if any.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: git receive-pack: support excluding objects from fsck'ing

The optional new config option `receive.fsck.skipList` specifies the path
to a file listing the names, i.e. SHA-1s, one per line, of objects that
are to be ignored by `git receive-pack` when `receive.fsckObjects = true`.

This is extremely handy in case of legacy repositories where it would
cause more pain to change incorrect objects than to live with them
(e.g. a duplicate 'author' line in an early commit object).

The intended use case is for server administrators to inspect objects
that are reported by `git push` as being too problematic to enter the
repository, and to add the objects' SHA-1 to a (preferably sorted) file
when the objects are legitimate, i.e. when it is determined that those
problematic objects should be allowed to enter the server.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>