[Editline] Redesign automatic indentation fix command for robustness

The FixIndentationCommand implementation has proven to be fragile across various libedit iterations. This patch reworks the command to use the same basic strategy as when moving between lines in a multi-line edit session: when indentation changes are required, exit line editing completely and restart with amended content. This approach won't be susceptible to subtle behavior differences libedit has introduced over time.

llvm-svn: 255548