lineedit: clean up tab completion code (variable reuse, comments)

lineedit: clean up tab completion code (variable reuse, comments)

Noted bugs in behavior.
Added debugging machinery.
Decoupled variables reused for unrelated purposes: apparently,
when not forced to use liveness analysis, gcc fares better.

function                                             old     new   delta
complete_cmd_dir_file                                699     705      +6
collapse_pos                                          75      79      +4
build_match_prefix                                   892     838     -54
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/1 up/down: 10/-54)            Total: -44 bytes

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>