fix(_known_hosts_real): remove empty hostnames (scop#737)

Fix the problem mentioned in Ref [1].  This is a regression caused by
commit 08dd2cd [2].  When there is an entry "Host *" in ssh_config,
an empty hostname is generated. We exclude the empty completions using
the compgen option "-X".

The empty hostname manifests typically with `scp <TAB>`, which before this
fix gave `scp :`.

[1] scop#720 (comment)
[2] \
scop@08dd2cd#diff-a4757074ff650000804fd3eaabe9b0a9e02e33040ca5b8afd4c0275fc5f3e136R1886

Author: akinomyoga

bash_completion

@@ -1924,7 +1924,7 @@ _known_hosts_real()
         local -a hosts=($(command sed -ne 's/^[[:blank:]]*[Hh][Oo][Ss][Tt][[:blank:]=]\{1,\}\(.*\)$/\1/p' "${config[@]}"))
         if ((${#hosts[@]} != 0)); then
             COMPREPLY+=($(compgen -P "$prefix" \
-                -S "$suffix" -W '"${hosts[@]%%[*?%]*}"' -X '\!*' -- "$cur"))
+                -S "$suffix" -W '"${hosts[@]%%[*?%]*}"' -X '@(\!*|)' -- "$cur"))
         fi
     fi
 

test/fixtures/_known_hosts_real/config

@@ -1,6 +1,8 @@
     UserKnownHostsFile _known_hosts_real/known_hosts
 
     # Unindented
+Host *
+     IPQoS none
 Host gee* jar?this-part-we-do-not-complete-at-least-yet
      HostName %h.example.com
 	# Indented, with = separator