fix(tar): fix access of the global variable basic_tar

The function `_gtar` calls `__tar_try_mode`, and `__tar_try_mode`
checks the state of the previous-scope `basic_tar`.  However,
`__tar_try_mode` called from `_gtar` references the state of the
global scope because `_gtar` does not declare `basic_tar`.  We need to
declare `basic_tar` local to `_gtar` the same as done in
`__posix_tar`.

Author: akinomyoga

completions/tar

@@ -486,11 +486,12 @@ __tar_detect_ext()
 
 _gtar()
 {
-    local long_opts short_opts \
+    local long_opts short_opts basic_tar \
         long_arg_none="" long_arg_opt="" long_arg_req="" \
         short_arg_none="" short_arg_opt="" short_arg_req="" \
         tar_mode tar_mode_arg old_opt_progress=0 \
         old_opt_used=0 old_opt_parsed=()
+    unset -v basic_tar # workaround for localvar_inherit
 
     # Main mode, e.g. -x or -c (extract/creation)
     local tar_mode=none