Optionally start with the tree panel toggled off

[amakrushin]

Side pane can be toggled with t. But there is no way of starting revdiff with side pane off.

Add an argument --no-tree.
Also, consider adding ini key.

revdiff --no-tree
```

[umputun]

right, no way to start with the tree hidden today, only the t toggle at runtime.

worth adding, it fits the existing pattern of --collapsed, --wrap, --line-numbers and friends. The ini key and REVDIFF_NO_TREE env var come with the flag for free.

if you want to bring a PR, go ahead. Otherwise I'll add it.

[amakrushin]

Please submit the PR yourself.

[umputun]

done in #308, merged to master.

--no-tree hides the file tree pane, with no-tree in the config file and REVDIFF_NO_TREE as the env var. t still brings the pane back, and on a single markdown file the TOC follows the same flag, matching what the toggle does today.

the layout math did not need touching: the resize path and the file-load path already branched on the same internal state the toggle sets, so the flag reuses them. The only real addition was moving initial focus to the diff pane when the tree starts hidden, otherwise the cursor keys would drive a pane that is not on screen.

this will be in the next release.