bash completion: work around PHP CLI readline in command substitution

Summary: (At least my build of) PHP tries to do readline magic on the terminal
when invoked from the shell, even inside bash's command substitution operator.
Work around it by piping the output of ##echo## to ##php##, so the input isn't a
terminal.

Test Plan: ##bash$ arc li<TAB>## now gives me possible completions rather than
breaking and inserting a literal tab

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 1096

resources/shell/bash-completion

@@ -1,24 +1,22 @@
-_arc()
+_arc ()
 {
-
   CUR="${COMP_WORDS[COMP_CWORD]}"
   COMPREPLY=()
-  OPTS=$(arc shell-complete --current ${COMP_CWORD} -- ${COMP_WORDS[@]})
-  
+  OPTS=$(echo | arc shell-complete --current ${COMP_CWORD} -- ${COMP_WORDS[@]})
+
   if [ $? -ne 0 ]; then
     return $?
   fi
-  
+
   if [ "$OPTS" = "FILE" ]; then
     COMPREPLY=( $(compgen -f -- ${CUR}) )
     return 0
   fi
-  
+
   if [ "$OPTS" = "ARGUMENT" ]; then
     return 0
   fi
-  
-  COMPREPLY=( $(compgen -W "${OPTS}" -- ${CUR}) )
 
+  COMPREPLY=( $(compgen -W "${OPTS}" -- ${CUR}) )
 }
 complete -F _arc -o filenames arc