The index version 4 introduced compressed path names for the entries.
From the git.git index-format documentation:

    At the beginning of an entry, an integer N in the variable width
    encoding [...] is stored, followed by a NUL-terminated string S.
    Removing N bytes from the end of the path name for the previous
    entry, and replacing it with the string S yields the path name for
    this entry.

But instead of stripping N bytes from the previous path's string and
using the remaining prefix, we were instead simply concatenating the
previous path with the current entry path, which is obviously wrong.

Fix the issue by correctly copying the first N bytes of the previous
entry only and concatenating the result with our current entry's path.

To calculate the path of a compressed index entry, we need to know the
preceding entry's path. While we do actually set the first predecessor
correctly to "", we fail to update this while reading the entries.

Fix the issue by updating `last` inside of the loop. Previously, we've
been passing a double-pointer to `read_entry`, which it didn't update.
As it is more obvious to update the pointer inside the loop itself,
though, we can simply convert it to a normal pointer.

The last written disk entry is currently being written inside of the
function `write_disk_entry`. Make behavior a bit more obviously by
instead setting it inside of `write_entries` while iterating all
entries.

Create a new function `index_entry_size` which encapsulates the logic to
calculate how much space is needed for an index entry, whether it is
simple/extended or compressed/uncompressed. This can later be re-used by
our code writing index entries.

Our code to write index entries to disk does not check whether the
entry that is to be written should use prefix compression for the path.
As such, we were overallocating memory and added bogus right-padding
into the resulting index entries. As there is no padding allowed in the
index version 4 format, this should actually result in an invalid index.

Fix this by re-using the newly extracted `index_entry_size` function.

All index entry size computations are now performed in
`index_entry_size`. As such, we do not need the file-scope macros for
computing these sizes anymore. Remove them and move the `entry_size`
macro into the `index_entry_size` function.

We have a check in place whether the index has enough data left for the
required footer after reading an index entry, but this was only used for
uncompressed entries. Move the check down a bit so that it is executed
for both compressed and uncompressed index entries.

When using compressed index entries, each entry's path is preceded by a
varint encoding how long the shared prefix with the previous index entry
actually is. We currently encode a length of `(path_len - same_len)`,
which is doubly wrong. First, `path_len` is already set to `path_len -
same_len` previously. Second, we want to encode the shared prefix rather
than the un-shared suffix length.

Fix this by using `same_len` as the varint value instead.

In our code writing index entries, we carry around a `disk_size`
representing how much memory we have in total and pass this value to
`git_encode_varint` to do bounds checks. This does not make much sense,
as at the time when passing on this variable it is already out of date.
Fix this by subtracting used memory from `disk_size` as we go along.
Furthermore, assert we've actually got enough space left to do the final
path memcpy.

The init and cleanup functions for test suites are usually prepended to
our actual tests. The index::version test suite does not adhere to this
stile. Fix this.

While we have a simple test to determine whether we can write an index
of version 4, we never verified that we are able to read this kind of
index (and in fact, we were not able to do so). Add a new repository
which has an index of version 4. This repository is then read from a new
test.

While we do have a test which checks whether a written index of version
4 has the correct version set, we do not check whether this actually
enables path compression for index entries. This commit adds a new test
by adding a number of index entries with equal path prefixes to the
index and subsequently flushing that to disk. With suffix compression
enabled by index version 4, only the last few bytes of these paths will
actually have to be written to the index, saving a lot of disk space.
For the test, differences are about an order of magnitude, allowing us
to easily verify without taking a deeper look at actual on-disk
contents.

The current write test does not trigger some edge-cases in the index
version 4 path compression code. Rewrite the test to start off the an
empty standard repository, creating index entries with interesting paths
itself. This allows for more fine-grained control over checked paths.
Furthermore, we now also verify that entry paths are actually
reconstructed correctly.