lack of progress indicator on merge button

[shiftkey]

Please describe the problem you think should be solved

After doing a bunch of testing for the upcoming 1.5 release, I've been missing an indicator that the merge is underway. For complex merges (where branches differ by 100s of commits) that take a while to complete, this means:

• you click the button to start the merge (whether in the compare tab or the "Merge into current branch" dialog)
• you wait an unclear amount of time while things are happening in the background
• the merge completes or you need to resolve conflicts

[Optional] Do you have any potential solutions in mind?

I like our pattern when committing to show the indeterminate progress indicator:

Also, we should disable the merge button to prevent multiple merges.

[shiftkey]

The new Merge Conflicts dialog could also be helped by this change:

[niik]

For future reference the merge conflicts dialog now has progress indication, see #10458

[steveward]

Requested in #11650.