uncommit: small cleanup of the interactive test setup

tests: add tests for interactively uncommitting by mentioning files

uncommit: fix the dirstate handling in `uncommit -i` This patch fixes the dirstate handling after `hg uncommit --interactive`. This patch is bit verbose about the logic and efforts are not put to plug the logic into existing logic as it's easier to understand like this. Plugging this into non-interactive logic deserves a separate patch.

uncommit: pre-indent a block in if-True The block will be moved in if-else in next patch.

uncommit: add an interactive option to uncommit This patchs adds an interactive flag to uncommit command which lets you choose hunks interactively. The implementation is as follows: 1) prompt user to select hunks to uncommit interactively 2) make a temporary commit of the hunks user selected 3) calculate the diff of temporary commit and changeset to be uncommitted 4) commit that diff which results in the changeset which should be there after uncommiting 5) create obsmarkers as required, phases and bookmarks handling 6) reroute the working directory

uncommit: pre-indent a block in if-True for the next patch

topic: provide more information when behind count cannot be computed The behind count has been confusing for various user and the associated error message is even worth. We try to clarify it to see if that helps.

topic: show the t0 even if topic is not yet touched

topics: improve the description if topic is not touched Instead of an empty stack in `hg stack` when topic does not contain any changeset, we show `not yet touched`.

topics: show working directory branch when topic is empty