gpg: use `which gpg2` for 'gpg.program' git configuration

This enables git-cola support TREZOR-based commit signatues,
together with "cola.signcommits=true" setting.

README-GPG.md

@@ -29,7 +29,7 @@ $ pip install --user git+https://github.com/romanz/trezor-agent.git
 ## Git commit & tag signatures:
 Git can use GPG to sign and verify commits and tags (see [here](https://git-scm.com/book/en/v2/Git-Tools-Signing-Your-Work)):
 ```
-$ git config --local gpg.program gpg2
+$ git config --local gpg.program $(which gpg2)
 $ git commit --gpg-sign                      # create GPG-signed commit
 $ git log --show-signature -1                # verify commit signature
 $ git tag --sign "v1.2.3"                    # create GPG-signed tag