Add gpg setting support

```
brew install gnupg gnupg2 pinentry-mac
git config --global user.signingkey
git config --global commit.gpgsign true
git config --global gpg.program gpg
```
Put the following in gpg.conf file (edit file with nano ~/.gnupg/gpg.conf command):

`no-tty`
Put the following in gpg-agent.conf file (edit file with nano ~/.gnupg/gpg-agent.conf command):

`pinentry-program /usr/local/bin/pinentry-mac`

Update:

You might need to execute killall gpg-agent command after editing the configurations file, gpg.conf, according to the comments. As the self-explanatory command says, this command will terminate the GPG (Gnu Privacy Guard) agent.

See also https://stackoverflow.com/questions/39494631/gpg-failed-to-sign-the-data-fatal-failed-to-write-commit-object-git-2-10-0

Review the issue’s Git commands and the referenced gpg.conf and gpg-agent.conf settings first. The request does not identify a Go file, entry point, test, or precise acceptance criteria; define how cgit should expose Git signing support and how the resulting configuration will be verified.

Written by the indexing model from the issue text.