win32: don't canonicalize symlink targets

Don't canonicalize symlink targets; our win32 path canonicalization
routines expect an absolute path.  In particular, using the path
canonicalization routines for symlink targets (introduced in commit
7d55bee6, "win32: fix relative symlinks pointing into dirs",
2020-01-10).

Now, use the utf8 -> utf16 relative path handling functions, so that
paths like "../foo" will be translated to "..\foo".

src/win32/posix_w32.c

@@ -447,8 +447,7 @@ int p_symlink(const char *target, const char *path)
 	 * relative symlinks, this is not someting we want.
 	 */
 	if (git_win32_path_from_utf8(path_w, path) < 0 ||
-	    git__utf8_to_16(target_w, MAX_PATH, target) < 0 ||
-	    git_win32_path_canonicalize(target_w) < 0)
+	    git_win32_path_relative_from_utf8(target_w, target) < 0)
 		return -1;
 
 	dwFlags = SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE;