topic: allow specifying current topic using period

Open Sushil Khanchi requested to merge topic/default/cmd-topic-feature into branch/default

This means hg topic . --rev <revspec> will amend the revs with current active topic. This would be useful in general.

Also, it can be provided as an "explicit" alternative to hg amend to change the topic of parent with current active topic.

Closes #44

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