Determines merge status in Show
Prints merge status in Formatter#format_issue
Fixes position of merge tag in Show

Looks like initially adding this through git commit -p didn't work out
as it should.
Only ask API for merge status when a PR is closed

A merged PR can never be open, therefore we don't have to call the API for
the merge status, unless the PR is closed.
Fix regression in format_issues_header

format_state silently acknowledged all nil state values as open, which
is not the case anymore. The issues header needs to declare the open
state now explicitly.
Led to dropping the green color that signifies open issues.