…subdirs

It is possible the user might actually just have a single build
directory rather than a hierarchy. We can handle this case by simply
adding the probed directory to the list if we detect any non-directory
files in the directory. While we are at it we shouldn't be returning
any non-directories so filter those out.

This ensures the most recent history items appears first on the
suggestions list.

We can't recover the current build directory when the user invokes M-i
so let's track it in a variable. The user still has to delete the
extra verbiage but at least won't be surprised about where the command
is run.