Fix magic completion

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This PR fixes an edge case in magic completions:

Currently:

If you press TAB after % or %% and select an item it doubles the percents

Example:

%<TAB> and select, say, %cd you'll get %%cd

After PR:

If you press TAB after % or %% and select an item it properly gives selection

Example:

%<TAB> and select, say, %cd you'll get %cd

[blink1073]

Thank you!