Remove dependency on sed(1) for history processing

We post-process `history 1`'s output to extract the current command.
This processing needs to strip the leading history number, an optional
`*` character indicating whether the entry was modified (or a space),
and then a space separating character.

We were previously using sed(1) for this, but we can implement an
equivalent transformation using bash's native parameter expansion
syntax.

bash-preexec.sh

@@ -250,10 +250,8 @@ __bp_preexec_invoke_exec() {
     fi
 
     local this_command
-    this_command=$(
-        export LC_ALL=C
-        HISTTIMEFORMAT='' builtin history 1 | sed '1 s/^ *[0-9][0-9]*[* ] //'
-    )
+    this_command=$(LC_ALL=C HISTTIMEFORMAT='' builtin history 1)
+    this_command="${this_command# *[[:digit:]]*[* ] }"
 
     # Sanity check to make sure we have something to invoke our function with.
     if [[ -z "$this_command" ]]; then