Offer a function to switch fsck errors to warnings and vice versa

Unfortunately, the FSCK_WARN/FSCK_ERROR flag is only really heeded by
git fsck, but other call paths (e.g. git index-pack --strict) error
out *always* no matter what type was specified. Therefore, we need
to take extra care to default to all FSCK_ERROR in those cases.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Allow demoting errors to warnings via receive.fsckobjects

The value can now be a list of options of the form '-missing_email'.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a test for non-boolean receive.fsckobjects values

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

Some kinds of errors are intrinsically unrecoverable (e.g. errors while
uncompressing objects). It does not make sense to allow demoting them to
mere warnings.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Make fsck_ident() warn-friendly

When fsck_ident() identifies a problem with the ident, it should still
advance the pointer to the next line so that fsck can continue in the
case of a mere warning.

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

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