Revisions display incorrect after switching to two handled mode, then back to single handled mode

[adamsilverstein]

To reproduce this bug:

• Create a post with several revisions
• Switch to two handled mode, drag the 'from' (left) handle to change the from comparison
• Switch back to single handle mode - the display is stuck on the two handle comparison, the from position should be changed to the revision previous to the to position, but it is not
• hitting reload without changing anything the display changes, it should not

Here is a screen recording demonstrating the issue: ​http://cl.ly/0Z1J1W2g2s3n

[adamsilverstein]

explicitly set from handle when switching back to single handled mode

[adamsilverstein]

in 26793.diff​ - reset the from model to one before the to model when switching back two handled mode to single handled mode.

[nacin]

26793.diff​ looks good to me at a glance. Good to go?

[adamsilverstein]

Replying to nacin:

26793.diff​ looks good to me at a glance. Good to go?

I think its fine and I tested it, I would still appreciate any additional review and testing.

[SergeyBiryukov]

In 29052:

Revisions: Explicitly set the 'from' handle position when switching from two-handled mode back to single-handled mode.

props adamsilverstein.
fixes #26793.