feat: support building for WASI

[msuozzo]

Build the vendored libgit2 for target_os = "wasi" (tested on
wasm32-wasip1; wasip2 is untested). There is no system libgit2 to link
against, so WASI always builds the vendored copy. In git2, path
conversions accept WASI's OsStr, and Repository::open_ext and
discover_path join ceiling directories themselves, since std's
env::join_paths is unsupported on WASI.

WASI preview 1 lacks several things libgit2's unix build assumes:

• mmap: build with NO_MMAP, so pack windows are read into memory.
• threads: leave GIT_THREADS undefined, and reject the -threads
  targets, where git2's Send types could then race inside libgit2.
• network connections: replace streams/socket.c with a stream that
  always fails, and reject the https and ssh features.
• user and process identity: wasi/compat.c defines getuid and friends
  (uid 0, pid 1). wasi-libc has no pwd.h, so wasi/include/pwd.h
  provides one; libgit2 only consults it when setuid, so the home
  directory comes from $HOME.

On wasm32, size_t is unsigned long but SIZE_MAX == UINT_MAX, so
libgit2's integer.h passes a size_t * to __builtin_uadd_overflow.
The types have the same width, so clang's error is downgraded for WASI
as a stopgap until libgit2 uses the type-generic builtins
(libgit2/libgit2#7406).

libz-sys before 1.1.30 builds zlib for WASI without default allocators
(rust-lang/libz-sys#273), so every object read fails. Require 1.1.30.

Add a CI job that builds for wasm32-wasip1 with wasi-sdk and runs the
tests under wasmtime. MSRV only builds: before Rust 1.94, std on WASI
can't canonicalize paths and its remove_dir_all fails on what the
tests create. For the tests:

• std has no temporary directory on WASI, so tests create theirs with
  test::tempdir(), which points tempfile at /tmp there, as tempfile's
  own tests do. The runner preopens /tmp.
• Test helpers with only unix and windows variants (realpath,
  ostr_to_cstr, file URLs, the symlink in repo_status) now cover
  WASI, and search_path joins paths itself.
• Skipped on WASI: compile_fail (trybuild runs cargo),
  non_utf8_branch (WASI paths are UTF-8), and two tests that remove
  directories through paths ending in a slash, which wasmtime rejects
  (wasip1 path_remove_directory fails with paths with trailing slashes bytecodealliance/wasmtime#11524).

[DanielEScherzer]

not going to pretend like I understand everything here, but some suggestions

View changes since this review

[DanielEScherzer]

not sure we should be reusing the GIT_UNUSED macro, maybe just void cast like the pwd stub?

[msuozzo]

GIT_UNUSED is already used elsewhere in the file as compat.c is built as part of libgit2 + depends on its internal headers anyway and so it follows libgit2's conventions. pwd.h is a libc stand-in, so it can't assume those headers, hence the difference.

[msuozzo]

I intend to incorporate the changes from #1316, which run the test suite on WASI, once libz-sys releases the fix they need (rust-lang/libz-sys#276).

[msuozzo]

libz-sys 1.1.30 is out, so I've folded in #1316. libgit2-sys now requires 1.1.30, and the WASI job runs the test suite under wasmtime.