The function `compute_write_order` may return a `NULL`-pointer
when an error occurs. In such cases we jump to the `done`-label
where we try to clean up allocated memory. Unfortunately we try
to deallocate the `write_order` array, though, which may be NULL
here.

Fix this error by returning early instead of jumping to the
`done` label. There is no data to be cleaned up anyway.

Without this change, compiling with gcc and pedantic generates warning:
ISO C does not allow extra ‘;’ outside of a function.

This is useful to send to the client while we're performing the work.

The reporting function has a force parameter which makes sure that we
do send out the message of 100% completed, even if this comes before the
next udpate window.

This function recursively inserts the given object and any referenced
ones. It can be thought of as a more general version of the functions to
insert a commit or tree.

Most use-cases for the object packer communicate in terms of commits
which each side has. We already have an object to specify this
relationship between commits, namely git_revwalk.

By knowing which commits we want to pack and which the other side
already has, we can perform similar optimisations to git, by marking
each tree as interesting or uninteresting only once, and not sending
those trees which we know the other side has.

Keep the definitions in the headers, while putting the declarations in
the C files. Putting the function definitions in headers causes
them to be duplicated if you include two headers with them.