Use correct IFS store/restore mechanism (#306)

The previous implementation had the problem that if IFS was not set
before, it was set to an empty string after restoring, which is not the
same as being unset. This broke grc / git forgit revert_commit, since
"git revert" was being interpreted as the command name instead of
command and argument.

The correct way is to check whether IFS is unset and, if so, unset it
again afterwards.

See for reference:
https://unix.stackexchange.com/a/264947/317320

bin/git-forgit

@@ -314,10 +314,11 @@ _forgit_cherry_pick() {
     [[ $fzf_exitval != 0 ]] && return $fzf_exitval
     [[ -z "$fzf_selection" ]] && return $fzf_exitval
 
-    old_IFS=$IFS IFS=$'\n' 
+    ${IFS+"false"} && unset old_IFS || old_IFS="$IFS"
+    IFS=$'\n'
     # shellcheck disable=2207
     commits=($(echo "$fzf_selection" | sort --numeric-sort --key=1 | cut -f2 | cut -d' ' -f1 | _forgit_reverse_lines))
-    IFS=${old_IFS}
+    ${old_IFS+"false"} && unset IFS || IFS="$old_IFS"
     [ ${#commits[@]} -eq 0 ] && return 1
 
     $git_cherry_pick "${commits[@]}"
@@ -553,15 +554,16 @@ _forgit_revert_commit() {
     files=$(sed -nE 's/.* -- (.*)/\1/p' <<< "$*") # extract files parameters for `git show` command
     preview="echo {} | cut -f2- | $_forgit_extract_sha | xargs -I% git show --color=always % -- $files | $_forgit_show_pager"
 
-    old_IFS=$IFS IFS=$'\n' 
+    ${IFS+"false"} && unset old_IFS || old_IFS="$IFS"
+    IFS=$'\n'
     # shellcheck disable=2207
     commits=($(eval "$cmd" | 
         nl |
         FZF_DEFAULT_OPTS="$opts" fzf --preview="$preview" -m | 
         sort --numeric-sort --key=1 | 
         cut -f2- |
         sed 's/^[^a-f^0-9]*\([a-f0-9]*\).*/\1/'))
-    IFS=${old_IFS}
+    ${old_IFS+"false"} && unset IFS || IFS="$old_IFS"
 
     [ ${#commits[@]} -eq 0 ] && return 1
 
@@ -607,11 +609,12 @@ _forgit_ignore() {
         --preview=\"eval $cmd\"
         $FORGIT_IGNORE_FZF_OPTS
     "
-    old_IFS=$IFS IFS=$'\n' 
+    ${IFS+"false"} && unset old_IFS || old_IFS="$IFS"
+    IFS=$'\n'
     # shellcheck disable=SC2206,2207
     args=($@) && [[ $# -eq 0 ]] && args=($(_forgit_ignore_list | nl -nrn -w4 -s'  ' |
         FZF_DEFAULT_OPTS="$opts" fzf | awk '{print $2}'))
-    IFS=${old_IFS}
+    ${old_IFS+"false"} && unset IFS || IFS="$old_IFS"
     [ ${#args[@]} -eq 0 ] && return 1
     # shellcheck disable=SC2068
     _forgit_ignore_get ${args[@]}