completion: avoid misleading completions in cone mode

The "set" and "add" subcommands of "sparse-checkout", when in cone mode,
should only complete on directories.  For bash_completion in general,
when no completions are returned for any subcommands, it will often fall
back to standard completion of files and directories as a substitute.
That is not helpful here.  Since we have already looked for all valid
completions, if none are found then falling back to standard bash file
and directory completion is at best actively misleading.  In fact, there
are three different ways it can be actively misleading.  Add a long
comment in the code about how that fallback behavior can deceive, and
disable the fallback by returning a fake result as the sole completion.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -3090,6 +3090,26 @@ __gitcomp_directories ()
 		# No possible further completions any deeper, so assume we're at
 		# a leaf directory and just consider it complete
 		__gitcomp_direct_append "$cur "
+	elif [[ $_found == 0 ]]; then
+		# No possible completions found.  Avoid falling back to
+		# bash's default file and directory completion, because all
+		# valid completions have already been searched and the
+		# fallbacks can do nothing but mislead.  In fact, they can
+		# mislead in three different ways:
+		#    1) Fallback file completion makes no sense when asking
+		#       for directory completions, as this function does.
+		#    2) Fallback directory completion is bad because
+		#       e.g. "/pro" is invalid and should NOT complete to
+		#       "/proc".
+		#    3) Fallback file/directory completion only completes
+		#       on paths that exist in the current working tree,
+		#       i.e. which are *already* part of their
+		#       sparse-checkout.  Thus, normal file and directory
+		#       completion is always useless for "git
+		#       sparse-checkout add" and is also probelmatic for
+		#       "git sparse-checkout set" unless using it to
+		#       strictly narrow the checkout.
+		COMPREPLY=( "" )
 	fi
 }