on `has_commit` check for the `backport` command.

I don't maintain local "master" branch on my ruby repository for stable
branch maintenance. I want just running `git fetch origin` to make it
work. It should work for those who pull origin/master into their local
master too.

with `done` command

The `rev && has_commit(rev, "ruby_#{TARGET_VERSION.tr('.','_')}")`
case seems to be used if and only if it's manually specified in the
argument to the done command. It was hard to notice it with the previous
code.

Some of the places in Redmine (e.g. Associated revisions) print
revisions using only 8 characters. Even when I copied a revision from
there, I want to prepend commit: in the message.

```
tool/redmine-backporter.rb:69: warning: literal string will be frozen in the future
```

from #backport_command_string

I don't want to leave unneeded spaces in the command history by
copy-pasting the entire line.