Without this change, any failed assertion in the second (or a later) test
inside a test suite has a chance of double deleting memory, resulting in
a heap corruption. See #1096 for details.

This leaves alone the test cases where we "just" use cl_git_sandbox_init()
and cl_git_sandbox_cleanup(). These methods already take good care to not
double delete a repository.

Fixes #1096

If you want to be absolutely safe with git_message_prettify, you
can now pass a NULL pointer for the buffer and get back the number
of bytes that would be copied into the buffer.

This means that an error is a non-negative return code and a
success will be greater than zero from this function.

This makes the message prettify buffer length check accurate.

git_commit() and git_tag() no longer prettify the
message by default. This has to be taken care of
by the caller.

This has the nice side effect of putting the
caller in position to actually choose to strip
the comments or not.

'git commit' and 'git tag -a' enforce some conventions, like cleaning up excess whitespace and making sure that the last line ends with a '\n'. This fix replicates this behavior.

Fix libgit2/libgit2sharp#117

Clay is the name of a programming language on the makings, and we want
to avoid confusions. Sorry for the huge diff!

Calling git_repository_open immediately after git_repository_init
results in memory leaks.