Add finish-issue to attach code to an open issue

This is the same as (hotfix|feature) finish but converts an existing
issue into a pull request rather than making a brand new one from
scratch.
Merge remote-tracking branch 'origin/master' into finish-issue
Merge remote-tracking branch 'origin/master' into finish-issue

Conflicts:
	man/feature-finish.1
Update pull description with last commit message

Like `finish`, `finish-issue` now constructs an updated pull description
using the last commit message and original issue body/title and prompts
the user to confirm the change.
finish-issue: Update man pages

The documentation needed updating now that we are modifying the
resulting pull request's description.
Merge remote-tracking branch 'origin/master' into finish-issue
finish-issue: Use require_argument, update help
Contribute: update completion.
Merge finish-issue (#74) into master

Add finish-issue to attach code to an open issue

This is the same as (hotfix|feature) finish but converts an existing
issue into a pull request rather than making a brand new one from
scratch.