[pass] Replace base64 in cygwin?

[Jason A. Donenfeld]

When we use the clipboard here's what happens:

1. The contents of the current clipboard is put in a temporary
variable for being restored 45 seconds later.
2. The first line of the desired file, or the newly generated
password, is copied to the clipboard, with no trailing newline.

Bash makes (1) very hard because it doesn't allow storing variables
with null characters, and makes it unnecessarily verbose for
remembering if the output of a command did or did not have a trailing
newline. For this reason, we instead treat the contents of the current
clipboard in (1) as generic binary data. As such, we store it
temporarily in base64 to work around bash's restrictions.