p_fallocate: add Windows emulation

Emulate `p_fallocate` on Windows by seeking beyond the end of the file
and setting the size to the current seek position.

src/win32/posix_w32.c

@@ -518,8 +518,54 @@ int p_creat(const char *path, mode_t mode)
 
 int p_fallocate(int fd, off_t offset, off_t len)
 {
-	error = ENOSYS;
-	return -1;
+	HANDLE fh = (HANDLE)_get_osfhandle(fd);
+	LARGE_INTEGER zero, position, oldsize, newsize;
+	size_t size;
+
+	if (fh == INVALID_HANDLE_VALUE) {
+		errno = EBADF;
+		return -1;
+	}
+
+	if (offset < 0 || len <= 0) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	if (git__add_sizet_overflow(&size, offset, len)) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	zero.u.LowPart = 0;
+	zero.u.HighPart = 0;
+
+	newsize.u.LowPart = (size & 0xffffffff);
+
+#if (SIZE_MAX > UINT32_MAX)
+	newsize.u.HighPart = size >> 32;
+#else
+	newsize.u.HighPart = 0;
+#endif
+
+	if (!GetFileSizeEx(fh, &oldsize)) {
+		set_errno();
+		return -1;
+	}
+
+	/* POSIX emulation: attempting to shrink the file is ignored */
+	if (oldsize.QuadPart >= newsize.QuadPart)
+		return 0;
+
+	if (!SetFilePointerEx(fh, zero, &position, FILE_CURRENT) ||
+	    !SetFilePointerEx(fh, newsize, NULL, FILE_BEGIN) ||
+	    !SetEndOfFile(fh) ||
+	    !SetFilePointerEx(fh, position, 0, FILE_BEGIN)) {
+		set_errno();
+		return -1;
+	}
+
+	return 0;
 }
 
 int p_utimes(const char *path, const struct p_timeval times[2])