Story: Show other branches in experiments webview

[aguschin]

Right now "Show experiments" webview doesn't show other branches but master:

It would be great to see other branches here as well, because sometimes we could be interested in comparing to one of them instead of master or current workspace version.

[rogermparent]

Great point!

Currently, we gather this table data with the default behavior of dvc exp show which only shows the currently checked out branch.

To address this issue, we'll have to think about how to allow users to optionally use exp show flags like -a to feed the table, for each user that wants to see all branches there'll be another that feels the table is cluttered or has so many experiments that their computer just can't handle showing every branch.

[mattseddon]

Closing in favour of #1966