This makes get_delta_base() return the error code as the return value
and the delta base as an out-parameter.

This change moves the responsibility of setting the error upon failures
of get_delta_base() to get_delta_base() instead of its callers. That
way, the caller chan always check if the return value is negative and
mark the whole operation as an error instead of using garbage values,
which can lead to crashes if the .pack files are malformed.

The file "sha1_lookup.c" contains a single function `sha1_position`
only which is used only in the packfile implementation. As the function
is comparatively small, to enable the compiler to optimize better and to
remove symbol visibility, move it into "pack.c".

While we do have a `git_zstream` abstraction that encapsulates all the
calls to zlib as well as its error handling, we do not use it in our
pack file code. Refactor it to make the code a lot easier to understand.

While we do have a zstream abstraction that encapsulates all the calls
to zlib as well as its error handling, we do not use it in our pack file
code. Refactor it to make the code a lot easier to understand.

Prefer `off64_t` internally.

Our file utils functions all have a "futils" prefix, e.g.
`git_futils_touch`. One would thus naturally guess that their
definitions and implementation would live in files "futils.h" and
"futils.c", respectively, but in fact they live in "fileops.h".

Rename the files to match expectations.

Currently, there is only one caller that adds entries into an offset map, and
this caller first uses `git_offmap_put` to add a key and then set the value at
the returned index by using `git_offmap_set_value_at`. This is just too tighlty
coupled with implementation details of the map as it exposes the index of
inserted entries, which we really do not care about at all.

Introduce a new function `git_offmap_set`, which takes as parameters the map,
key and value and directly returns an error code. Convert the caller to make use
of it instead.

The current way of looking up an entry from a map is tightly coupled with the
map implementation, as one first has to look up the index of the key and then
retrieve the associated value by using the index. As a caller, you usually do
not care about any indices at all, though, so this is more complicated than
really necessary. Furthermore, it invites for errors to happen if the correct
error checking sequence is not being followed.

Introduce a new high-level function `git_offmap_get` that takes a map and a key
and returns a pointer to the associated value if such a key exists. Otherwise,
a `NULL` pointer is returned. Adjust all callers that can trivially be
converted.