`gpg.program` treats a leading ‘~’ literally, unlike other path
options such as `core.excludesfile`.
Error:
fatal: cannot exec '~/.local/bin/mygpg': No such file or directory
Reproducer:
ln -s "$(command -v gpg)" ~/.local/bin/mygpg
git config gpg.program '~/.local/bin/mygpg'
git commit -S -m "test" # fails as above
Expected: Git should expand ‘~’ to $HOME.
Work-around: have to use an absolute path
