While the tests in object::tag::read exercises reading and parsing valid
tags from the ODB, they barely try to verify that the parser fails in a
sane way when parsing invalid tags. Create a new test suite
object::tag::parse that directly exercise the parser by using
`git_object__from_raw` and add various tests for valid and invalid tags.

When parsing tags, we skip all unknown fields that appear before the tag
message. This skipping is done by using a plain `strstr(buffer, "\n\n")`
to search for the two newlines that separate tag fields from tag
message. As it is not possible to supply a buffer length to `strstr`,
this call may skip over the buffer's end and thus result in an out of
bounds read. As `strstr` may return a pointer that is out of bounds, the
following computation of `buffer_end - buffer` will overflow and result
in an allocation of an invalid length.

Fix the issue by using `git__memmem` instead. Add a test that verifies
parsing the tag fails not due to the allocation failure but due to the
tag having no message.

We currently do not have any test suites dedicated to parsing commits
from their raw representations. Add one based on `git_object__from_raw`
to be able to test special cases more easily.

The commit message encoding is currently being parsed by the
`git__prefixcmp` function. As this function does not accept a buffer
length, it will happily skip over a buffer's end if it is not `NUL`
terminated.

Fix the issue by using `git__prefixncmp` instead. Add a test that
verifies that we are unable to parse the encoding field if it's cut off
by the supplied buffer length.