Remove the signature from ref-modifying functions

The signature for the reflog is not something which changes
dynamically. Almost all uses will be NULL, since we want for the
repository's default identity to be used, making it noise.

In order to allow for changing the identity, we instead provide
git_repository_set_ident() and git_repository_ident() which allow a user
to override the choice of signature.

describe example: function to add commits to opts

Add safe function to (a,rea)llocate memory which terminate example on
memory allocation failure.

Move code to allocate commits to its own function.

I don't get how it was working without git_libgit2_init() call. I run it
and libgit2 throws assertion somewhere in its internals. Now it works.

Updated commit with shutdown at the end.

We need to allocate memory for sizeof(char *) * ncommits, not just for
ncommits.

Issue detected by GCC's AddressSanitizer.

Local transports don't have data about the size, avoid dividing by
zero in the callback.