Start the diff view directly from UI, not just via --diff in CLI

[Nindaleth]

As originally stated in #26599 (comment), I see a nice QoL enhancement for the file difference feature:

I'm asking because in some cases it's faster use the UI to open two files than to switch to terminal and search there:

• when the files are somewhere deep in a git tree that can be opened as a project
• when one or both buffers are unsaved (copypasted from elsewhere)

This could also have been in "Ideas from Sublime Text" section, but seeing the core of the feature already implemented in Zed, I'm filing it as enhancement.

In Sublime Text the diff can be started from UI like this:

• have at least two buffers open, select one tab
• right click another tab
• click "Diff with current tab..."

NOTE: just in case - this is not git diff, but text diff