Fix the problem of key bindings in finder buffers

[Shuangcheng-Ni]

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Why this change is necessary and useful

As we discussed on gitter, some extra insert-mode key bindings might dismiss the finder buffer. For example, if a user has something like inoremap <BS> <C-r>=auto_pair()<CR> in his/her .vimrc, the finder buffer would immediately disappear after pressing <BS>.
This commit fixes the problem. It tries to override all the global mappings with some buffer-local mappings that remap each previously mapped key sequence to itself in the finder buffer, while the previously defined buffer-local mappings in the finder buffer are not overridden.

[Please explain in detail why the changes in this PR are needed.]