git-credential-netrc: accept gpg option

git-credential-netrc: accept gpg option

git-credential-netrc was hardcoded to decrypt with 'gpg' regardless of
the gpg.program option. This is a problem on distributions like Debian
that call modern GnuPG something else, like 'gpg2'.
Set the command according to these settings in descending precedence
1. the git-credential-netrc command -g|--gpg option
2. the git gpg.program configuration option
3. the default: 'gpg'

For conformance with Documentation/CodingGuidelines
- use Git.pm for repository and global option queries
- document -g|--gpg command option in command usage
- test repository & command options
- write documentation placeholders according to main standards

Signed-off-by: Luis Marsano <luis.marsano@gmail.com>
Acked-by: Ted Zlatanov <tzz@lifelogs.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>