Improvements for 'Detached HEAD' state

[srbs]

Problem description

For when users accidentally or purposefully enter into a detached HEAD state (via SM or command line), Sublime Merge should indicate this and behave better in standard operations.

Preferred solution

Here are some issues I have found:

• there should be a visual indicator to show when you are in a non-normal state
  • similar to how git status shows HEAD detached at in red
• pull is invalid, should be hidden or restricted & defaulted to fetch
• checkout when unconnected commits exist, some confirmation is needed due to losing those commits
  • note: SM uses git checkout -q which prevents users from fixing the problem without diving into the ref logs