When min_chars is nonzero, if the first command that opens the
completion has < min_chars on the word under the cursor, it triggers a
check for a condition where last_cursor_pos is None.
By setting last_cursor_pos=-1 we ensure that the completer always
updates the first time it is opened, and that there is never a check
against None.
This adds a test for the min_chars feature.
Resolves#3635.
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