feat(_comp_compgen_filedir{,_xspec}): do not generate "." and ".."

Currently, "." and ".." are generated as completions when the current
path segment starts with a dot.  However, this impedes the completion
of any dotfiles when the current path segments is "." because the word
"." already matches the generated ".".  When the user attempts the
completion after inputting ".", the user is likely to intend to
complete a dotfile name as ".dotfile" (rather than to complete just a
slash as "./" because the user could just press "/" in such a case).
In this patch, we do not generate "." and ".." unless the user has
explicitly input "..".

The behavioral changes are summarized below.  Other possibilities of
the detailed behaviors are commented in the "[ Note: ... ]" sections.
If necessary, they can be reconsidered and adjusted in later commits.

* cmd .[TAB]

  When the current directory has no dotfiles (i.e., filenames starting
  with "."), it completed a slash.  Nothing will happen after this
  patch. [ Note: as another option, we might generate "." only when no
  dotfiles are found.  However, that might be annoying when the user
  tries to probe the existence of the dotfiles by pressing TAB, where
  the user does not expect the insertion of a slash. ]

  When the current directory has dotfiles, nothing happened.  After
  this patch, this will insert the common prefix of the dotfiles.
  Note that both "." and ".." are ignored in determining the common
  prefix.

* cmd ..[TAB]

  When the current directory has no files starting with "..", this
  completes a slash to form "../".  The behavior will not be changed
  by this patch. [ Note: as another option, we might disable ".." at
  all to be consistent with the case of ".".  However, the files
  starting with ".." are unlikely, and the user is less likely to
  probe the existence of the files starting with ".." by pressing TAB
  after "..".  For this reason, we generate ".." even if it would
  prevent completion of the common prefix of the files. ]

  When the current directory has files starting with "..", nothing
  happens with this.  The behavior will not be changed by this
  patch. [ Note: as another option, we might generate ".." only when
  there are no files starting with "..", but we here assume that the
  user may want to complete a slash as ".." even when there are files
  starting with "..". ]

References:
#364
#1230

Co-authored-by: Koichi Murase <myoga.murase@gmail.com>

bash_completion

@@ -1142,6 +1142,15 @@ _comp_compgen_filedir()
                 -f ${_plusdirs+"${_plusdirs[@]}"}
     fi
 
+    if ((${#toks[@]} != 0)); then
+        # Remove . and .. (as well as */. and */..) from suggestions, unless
+        # .. or */.. was typed explicitly by the user (for users who use
+        # tab-completion to append a slash after '..')
+        if [[ $cur != ?(*/).. ]]; then
+            _comp_compgen -Rv toks -- -X '?(*/)@(.|..)' -W '"${toks[@]}"'
+        fi
+    fi
+
     if ((${#toks[@]} != 0)); then
         # 2>/dev/null for direct invocation, e.g. in the _comp_compgen_filedir
         # unit test
@@ -3042,6 +3051,13 @@ _comp_compgen_filedir_xspec()
 
     ((${#toks[@]})) || return 1
 
+    # Remove . and .. (as well as */. and */..) from suggestions, unless .. or
+    # */.. was typed explicitly by the user (for users who use tab-completion
+    # to append a slash after '..')
+    if [[ $cur != ?(*/).. ]]; then
+        _comp_compgen -Rv toks -- -X '?(*/)@(.|..)' -W '"${toks[@]}"' || return 1
+    fi
+
     compopt -o filenames
     _comp_compgen -RU toks -- -W '"${toks[@]}"'
 }

test/t/unit/test_unit_compgen_filedir.py

@@ -6,7 +6,7 @@
 
 import pytest
 
-from conftest import assert_bash_exec, assert_complete
+from conftest import assert_bash_exec, assert_complete, bash_env_saved
 
 
 @pytest.mark.bashcomp(cmd=None, ignore_env=r"^\+COMPREPLY=")
@@ -59,7 +59,9 @@ def utf8_ctype(self, bash):
         return lc_ctype
 
     def test_1(self, bash):
-        assert_bash_exec(bash, "_comp_compgen_filedir >/dev/null")
+        with bash_env_saved(bash) as bash_env:
+            bash_env.write_variable("cur", "")
+            assert_bash_exec(bash, 'cur="" _comp_compgen_filedir >/dev/null')
 
     @pytest.mark.parametrize("funcname", "f f2".split())
     def test_2(self, bash, functions, funcname):
@@ -233,3 +235,27 @@ def test_26(self, bash, functions, funcname):
     def test_27(self, bash, functions, funcname, utf8_ctype):
         completion = assert_complete(bash, "%s aé/" % funcname, cwd="_filedir")
         assert completion == "g"
+
+    @pytest.mark.parametrize("funcname", "f f2".split())
+    def test_28_dot_1(self, bash, functions, funcname):
+        """Exclude . and .. when the completion is attempted for '.[TAB]'"""
+        completion = assert_complete(bash, r"%s ." % funcname, cwd="_filedir")
+        assert completion == [".dotfile1", ".dotfile2"]
+
+    @pytest.mark.parametrize("funcname", "f f2".split())
+    def test_28_dot_2(self, bash, functions, funcname):
+        """Exclude . and .. when the completion is attempted for 'dir/.[TAB]'"""
+        completion = assert_complete(bash, r"%s _filedir/." % funcname)
+        assert completion == [".dotfile1", ".dotfile2"]
+
+    @pytest.mark.parametrize("funcname", "f f2".split())
+    def test_28_dot_3(self, bash, functions, funcname):
+        """Include . when the completion is attempted for '..[TAB]'"""
+        completion = assert_complete(bash, r"%s .." % funcname, cwd="_filedir")
+        assert completion == "/"
+
+    @pytest.mark.parametrize("funcname", "f f2".split())
+    def test_28_dot_4(self, bash, functions, funcname):
+        """Include . when the completion is attempted for '..[TAB]'"""
+        completion = assert_complete(bash, r"%s _filedir/.." % funcname)
+        assert completion == "/"