Provide a function to parse an fsck message type

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Offer a function to switch fsck errors to warnings and vice versa

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>
WIP: Add a test for non-boolean receive.fsckobjects values

TODO!!! This only *happens* to work because the fsck machinery is
ill-prepared to handle warnings. It is based on the concept that we can
simply error out when there is so much as even a warning, but what we want
to do is to try to figure things out and continue even after detecting
that, say, an email is missing or invalid.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
TODO: fixup t5504

This demonstrates what needs to work, and does not yet.

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