Make a distinction between generated headers and "translated" headers.
This is important to support build-time dependencies when headers are
updated.
Generated headers are those which contain build-time feature
specifications, like `git2_features.h` that are internal to the build
and `experimental.h` that contain API information.
Translated headers are the headers that are in `include/git2`, but may
be translated to have a unique prefix like `incklude/git2-experimental`.
This distinction is important so that the CMakeFiles.txt depend on the
in-tree include files (`src/include`) and the generated header files
_but not_ the translated header files. Otherwise there are two `pack.h`
and it's unclear whether the in-tree build is targeting the one in
`src/include` or the one in the build tree.
Without this, updating an in-tree header file like `pack.h` would not
cause a rebuild of its dependencies.
While the reftable library is mostly decoupled from the Git codebase,
some specific functionality intentionally taps into Git subsystems. To
make porting of the reftable library easier all of these are contained
in "system.h" and "system.c".
Reimplement those compatibility shims so that they work for libgit2.
Import the reftable library from commit 4fee6ff3b2 (Merge branch
'ps/reftable-portability', 2026-04-08). This is an exact copy of the
reftable library.
The library will be wired into libgit2 over the next couple of commits.
