decrypting the existing one if the filename or path contains pairs
of square brackets
vim-gnupg uses glob to check if a file exists but glob expands wildcards (including [ and ]) so, the check returns 'false' even if the file exists if the filename or path is something like -Directory/File.
Using filereadable avoids this problem.
Fixes a problem that made vim-gnupg open an emtpy file instead of
Looks good. I'll merge it soon.