It is all fine and dandy when users GPG-sign files from the terminal.
But when they do not, things get more complicated.
The usual thing is to install a pinentry helper targeting your graphical
desktop and then launching the GPG agent somehow and connet it all
together.
Except that MSYS2 does not come with a gpg-agent.
So let's just imitate openssh, which interprets the environment variable
SSH_ASKPASS (if set) as the path to a helper that can be called upon to
ask the user to provide a passphrase.
Unsurprisingly, the new environment variable is called GPG_ASKPASS (and
we do not repeat Git's mistake of co-opting SSH_ASKPASS, if set).
This allows, say, Git for Windows' Git GUI to commit with GPG signatures
even if the passphrase is not empty.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
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