Before, pressing <Enter> when completions were visible would execute the
selected completion. As soon as completions were provided, the first
item would be selected. This could cause issues e.g. when changing
folders:
:cf <Enter>
Previously, this would have selected the first folder in the list. Now,
since <Tab>, <C-n>, etc have not been pressed to select the first
completion, the command above simply executes `:cf `.
To accomplish this, a "no-op completion" has been added at index -1.
07a9b9204e