Public header git2/sys/stream.h uses ssize_t, undefined for MSVC/clang-cl consumers

从 git2/sys/stream.h 中的公共声明开始，并将其类型处理方式与 src/util/win32/msvc-compat.h 中的内部定义进行比较。使用 MSVC 或 clang-cl consumer 验证该 header，然后确认包含它能够提供 ssize_t，且不会破坏其他平台，同时确认 stream callback 声明可以成功编译。

Summary

The public header git2/sys/stream.h declares the git_stream read/write callbacks using ssize_t (v1.9.4, lines 59–60):

ssize_t GIT_CALLBACK(read)(struct git_stream *, void *, size_t);
ssize_t GIT_CALLBACK(write)(struct git_stream *, const char *, size_t, int);

ssize_t is POSIX, not ISO C, and is not provided by MSVC / clang-cl. The only definition in the tree is the internal src/util/win32/msvc-compat.h (typedef SSIZE_T ssize_t;), which is not part of the public/installed headers. So a Windows consumer that includes git2/sys/stream.h — e.g. to register a custom stream backend via git_stream_register — fails to compile, e.g. with clang-cl:

error: unknown type name 'ssize_t'

git2.h does not pull in sys/stream.h, so this only affects consumers using the stream/transport plugin API directly — but for those it's a hard compile error on Windows with no public type to fall back on.

Suggested fix

Make ssize_t available from a public header on Windows before it's used (in git2/sys/stream.h, or a small shared public compat shim), mirroring what msvc-compat.h already does internally:

#if defined(_WIN32) && !defined(__MINGW32__) && !defined(_SSIZE_T_DEFINED)
# include <BaseTsd.h>
typedef SSIZE_T ssize_t;
# define _SSIZE_T_DEFINED
#endif

Happy to send a PR if you'd like.