Rebase with unstaged changes should have clear prompt

[avit]

The rebase command shows an error dialog with the long, unformatted list of files when there are unstaged changes. The actual reason for the error is not shown clearly, except at the end of the raw command output, which can be hidden if there are many files.

The last line of the git command output says "cannot rebase: you have unstaged changes". This should be in the bold text of the error title, instead of the unclear "Rebase failed!"

The output of the git command with the list of unstaged files could be omitted by changing the dialog to use buttons:

• Cancel (return to branch view)
• Stage Files (default button: go to stage view)

[brotherbard]

Currently the error dialogs are generic and just show whatever git spits out. This was done to get things working quickly. Eventually I will go back and improve the messages but it's a pretty low priority at the moment.