fix(mdtool): use -f -X '!pat' instead of -G pat

#1297 (comment)

The compgen option `-X '!pat'` has a problem that it only generates
the filenames in the current directory and that it does not perform
the filtering by the "cur" specified to the argument of `compgen`:

    $ compgen -G '*.md' -- doc/
    CHANGELOG.md
    CONTRIBUTING.md
    README.md
    style.md

In this patch, we use `-f -X '!pat'`, which does not have those
limitations.

Author: akinomyoga

CONTRIBUTING.md

@@ -132,6 +132,14 @@ Also, please bear the following coding guidelines in mind:
   expansions will be unexpectedly performed, which becomes a vulnerability.  In
   the latter case, checks by shellcheck and shfmt will not be performed inside
   `'...'`.  Also, `_comp_compgen_split` is `IFS`-safe.
+  
+  Avoid using `_comp_compgen -- -G "pattern"` to generate completions.  The
+  result is not filtered by the current word `cur` due to the Bash design of
+  `compgen`.  Also, this cannot be used to generate filenames with a specified
+  extension because the `-G` specification only generates the matching
+  filepaths in the current directory.  It does not look into subdirectories
+  even when `$cur` implies the completion in a subdirectory.  One can instead
+  use `_comp_compgen -- -f -X '!pattern'`.
 
 - When completing available options, offer only the most descriptive
   ones as completion results if there are multiple options that do the

completions/mdtool

@@ -21,18 +21,18 @@ _comp_cmd_mdtool()
                 #if [[ "$prev" == *: ]]; then
                 #   case $prev in
                 #       @(--p:|--project:))
-                #           _comp_compgen -- -G "*.mdp"
+                #           _comp_compgen -- -f -X '!*.mdp'
                 #           ;;
                 #       @(--f:|--buildfile:))
-                #           _comp_compgen -- -G "*.md[ps]"
+                #           _comp_compgen -- -f -X '!*.md[ps]'
                 #           ;;
                 #   esac
                 #fi
                 return
                 ;;
             "generate-makefiles")
                 compopt -o filenames
-                _comp_compgen -- -o filenames -G"*.mds"
+                _comp_compgen -- -o filenames -f -X '!*.mds'
                 if [[ $prev == *mds ]]; then
                     _comp_compgen -- -W '--simple-makefiles --s --d:'
                 fi