msys2_path_conv: pass PC_NOFULL to path_conv

In theory this doesn't make a difference because posix_to_win32_path()
is only called with rooted/absolute paths, but as pointed out in
#103 PC_NOFULL will preserve
the trailing slash of unix paths (for some reason).

See "cygpath -m /bin/" (preserved) vs "cygpath -am /bin/" (dropped)

One use case where we need to trailing slashes to be preserved is the GCC build
system:
https://github.com/gcc-mirror/gcc/blob/6d82e0fea5f988e829912a/gcc/Makefile.in#L2314

The Makefile appends a slash to the prefixes and the C code doing relocation will
treat the path as a directory if there is a trailing slash. See
msys2/MINGW-packages#14173 for details.

With this change all our MSYS2 path_conv tests pass again.

winsup/cygwin/msys2_path_conv.cc

@@ -682,7 +682,7 @@ void posix_to_win32_path(const char* from, const char* to, char** dst, const cha
         strncpy(one_path, from, to-from);
         one_path[to-from] = '\0';
 
-        path_conv conv (one_path, 0);
+        path_conv conv (one_path, PC_NOFULL);
         if (conv.error)
         {
           set_errno(conv.error);