Conflict Solver: ask if HEAD commit has changed

Maybe not close them, but just warn that Conflict Solver state is no longer in sync with working tree.

Here's how current behavior can cause problems:

1) Start rebasing
2) Run into conflict
3) Open conflict solver
4) Open many more windows to figure how to solve conflict (yes, some conflicts require quite an investigation to solve!)
5) Forget you have conflict solver open
6) Open another conflict solver
7) Solve conflicts
8) Continue rebase
9) Run into new conflicts
10) While alt-tabbing through open windows by accident open an outdated Conflict Solver

11) This is quite confusing. Also, if you actually save now, you will overwrite file with an outdated version of it.