fsck: allow upgrading fsck warnings to errors

The 'invalid tag name' and 'missing tagger entry' warnings can now be
upgraded to errors by setting receive.fsck.invalid-tag-name and
receive.fsck.missing-tagger-entry to 'error'.

Incidentally, the missing tagger warning is now really shown as a warning
(as opposed to being reported with the "error:" prefix, as it used to be
the case before this commit).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Document the new receive.fsck.* options.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fsck: support demoting errors to warnings

There are some legacy repositories out there which display non-fatal
errors with fsck. Let's make fsck useful with such repositories by
allowing users to ignore known issues, or at least demote those issues
to mere warnings (e.g. when a very early, hand-crafted commit has two
authors instead of one).

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

This option avoids unpacking each and all objects, and just verifies
the connectivity.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
WIP: Introduce receive.fsck.skip-list

This config option can be used to specify the path to a file whose
lines list names of objects that should be ignored when fsck'ing
incoming objects.

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).

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