Fix problem with minibuffer commands.

When executing the pre-command hook, (minibufferp) is not yet t; in
post-command-hook it is no longer t.  This means that typing e.g. M-x
ignore RET will cause the current line to be reindented, because
post-command-hook thinks that the last command was (non-minibuffer)
return.  Thus, we should set
auto-indent-last-pre-command-hook-minibufferp whenever the minibuffer is
entered, and not just in the pre-command hook.

One can observe this issue by opening a buffer with auto-indent-mode on,
changing the indentation of a line to be incorrect, then running M-x
ignore; the line should reindent.  In practice, this is an issue for
python code like the following:

    if condition:
        do_something()
    
    something_else()

Executing a minibuffer command with point on the something_else line
will cause that line to be indented into the conditional.