We've accumulated quite some functions which are never used outside of
their respective code unit, but which are lacking the `static` keyword.
Add it to reduce their linkage scope and allow the compiler to optimize
better.

git_packbuilder_write: Allow setting path to NULL to use the default path

Propagate failures caused by pool initialization errors.

If given a NULL path, write to the object path of the repository.

Add tests for the new behavior.

Clean up and return via a single label, to avoid duplicate error
handling before each return, and to make it easier to extend the set of
cleanups needed.

While `git_zstream_set_input` cannot fail right now, it might change in
the future if we ever decide to have it check its parameters more
vigorously. Let's thus check whether its return code signals an error.

`cvar` is an unhelpful name.  Refactor its usage to `configmap` for more
clarity.

The packbuilder code allocates memory in chunks. When it needs to
allocate, it tries to add 1024 to the number of objects and multiply by
3/2. However, it actually multiplies by 1 instead, since it performs an
integral division in the expression "3 / 2" and only then multiplies by
the increased number of objects.

The current behavior causes the code to waste massive amounts of time
copying memory when it reallocates, causing inserting all non-blob
objects in the Linux repository into a new pack to take some
indeterminate time greater than 5 minutes instead of 52 seconds.

Correct this error by first dividing by two, and only then multiplying
by 3. We still check for overflow for the multiplication, which is the
only part that can overflow. This appears to be the only place in the
code base which has this problem.

Update internal usage of `git_transfer_progress` to
`git_indexer_progreses`.

Currently, one would use either `git_oidmap_insert` to insert key/value pairs
into a map or `git_oidmap_put` to insert a key only. These function have
historically been macros, which is why their syntax is kind of weird: instead of
returning an error code directly, they instead have to be passed a pointer to
where the return value shall be stored. This does not match libgit2's common
idiom of directly returning error codes.Furthermore, `git_oidmap_put` is tightly
coupled with implementation details of the map as it exposes the index of
inserted entries.

Introduce a new function `git_oidmap_set`, which takes as parameters the map,
key and value and directly returns an error code. Convert all trivial callers of
`git_oidmap_insert` and `git_oidmap_put` to make use of it.