[bash] Refactor access to "_fzf_orig_complete_${cmd//[^A-Za-z0-9_]/_}"

In the current codebase, for the original completion settings, the
pieces of the codes to determine the variable name and to access the
stored data are scattered.  In this patch, we define functions to
access these variables.  Those functions will be used in a coming
patch.

* This patch also resolves an inconsistent escaping of "$cmd": $cmd is
  escaped as ${...//[^A-Za-z0-9_]/_} in some places, but it is escaped
  as ${...//[^A-Za-z0-9_=]/_} in some other places.  The latter leaves
  the character "=" in the command name, which causes an issue because
  "=" cannot be a part of a variable name.  For example, the following
  test case produces an error message:

  $ COMP_WORDBREAKS=${COMP_WORDBREAKS//=}
  $ _test1() { COMPREPLY=(); }
  $ complete -vF _test1 cmd.v=1.0
  $ _fzf_setup_completion path cmd.v=1.0
  $ cmd.v=1.0 [TAB]
  bash: _fzf_orig_completion_cmd_v=1_0: invalid variable name

  The behavior of leaving "=" was present from the beginning when
  saving the original completion is introduced in commit 91401514, and
  this does not seem to be a specific reasoning.  In this patch, we
  replace "=" as well as the other non-identifier characters.

* Note: In this patch, the variable REPLY is used to return values
  from functions.  This design is to make it useful with the value
  substitutions, a new Bash feature of the next release 5.3, which is
  taken from mksh.

shell/completion.bash

@@ -63,6 +63,31 @@ __fzf_orig_completion() {
   done
 }
 
+# @param $1 cmd - Command name for which the original completion is searched
+# @var[out] REPLY - Original function name is returned
+__fzf_orig_completion_get_orig_func() {
+  local cmd orig_var orig
+  cmd=$1
+  orig_var="_fzf_orig_completion_${cmd//[^A-Za-z0-9_]/_}"
+  orig="${!orig_var-}"
+  REPLY="${orig##*#}"
+  [[ $REPLY ]] && type "$REPLY" &> /dev/null
+}
+
+# @param $1 cmd - Command name for which the original completion is searched
+# @param $2 func - Fzf's completion function to replace the original function
+# @var[out] REPLY - Completion setting is returned as a string to "eval"
+__fzf_orig_completion_instantiate() {
+  local cmd func orig_var orig
+  cmd=$1
+  func=$2
+  orig_var="_fzf_orig_completion_${cmd//[^A-Za-z0-9_]/_}"
+  orig="${!orig_var-}"
+  orig="${orig%#*}"
+  [[ $orig == *' %s '* ]] || return 1
+  printf -v REPLY "$orig" "$func"
+}
+
 _fzf_opts_completion() {
   local cur prev opts
   COMPREPLY=()
@@ -261,15 +286,12 @@ _fzf_opts_completion() {
 }
 
 _fzf_handle_dynamic_completion() {
-  local cmd orig_var orig ret orig_cmd orig_complete
+  local cmd ret REPLY orig_cmd orig_complete
   cmd="$1"
   shift
   orig_cmd="$1"
-  orig_var="_fzf_orig_completion_$cmd"
-  orig="${!orig_var-}"
-  orig="${orig##*#}"
-  if [[ -n "$orig" ]] && type "$orig" > /dev/null 2>&1; then
-    $orig "$@"
+  if __fzf_orig_completion_get_orig_func "$cmd"; then
+    "$REPLY" "$@"
   elif [[ -n "${_fzf_completion_loader-}" ]]; then
     orig_complete=$(complete -p "$orig_cmd" 2> /dev/null)
     _completion_loader "$@"
@@ -293,7 +315,6 @@ __fzf_generic_path_completion() {
   if [[ $cmd == \\* ]]; then
     cmd="${cmd:1}"
   fi
-  cmd="${cmd//[^A-Za-z0-9_=]/_}"
   COMPREPLY=()
   trigger=${FZF_COMPLETION_TRIGGER-'**'}
   cur="${COMP_WORDS[COMP_CWORD]}"
@@ -359,7 +380,6 @@ _fzf_complete() {
   post="$(caller 0 | command awk '{print $2}')_post"
   type -t "$post" > /dev/null 2>&1 || post='command cat'
 
-  cmd="${COMP_WORDS[0]//[^A-Za-z0-9_=]/_}"
   trigger=${FZF_COMPLETION_TRIGGER-'**'}
   cur="${COMP_WORDS[COMP_CWORD]}"
   if [[ "$cur" == *"$trigger" ]] && [[ $cur != *'$('* ]] && [[ $cur != *':='* ]] && [[ $cur != *'`'* ]]; then
@@ -488,15 +508,12 @@ if type _completion_loader > /dev/null 2>&1; then
 fi
 
 __fzf_defc() {
-  local cmd func opts orig_var orig def
+  local cmd func opts REPLY
   cmd="$1"
   func="$2"
   opts="$3"
-  orig_var="_fzf_orig_completion_${cmd//[^A-Za-z0-9_]/_}"
-  orig="${!orig_var-}"
-  if [[ -n "$orig" ]]; then
-    printf -v def "$orig" "$func"
-    eval "$def"
+  if __fzf_orig_completion_instantiate "$cmd" "$func"; then
+    eval "$REPLY"
   else
     complete -F "$func" $opts "$cmd"
   fi