[tib8]

A new separate tree column, so the message texts are aligned vetically:
(some related PRs also visible on this screenshot)

The history graph currently shares a column with commit messages, so message indentation changes with the number of graph lanes. Add an optional TREE item above AUTHOR in the advanced options menu's SHOW COLUMNS group. When enabled, the graph moves into a separate first column and the message rows align vertically. The option defaults to off and is saved per repository.

The TREE column automatically expands and shrinks to fit the graph lanes in the visible rows as the user scrolls. Its header text clips when necessary and does not impose a minimum width. Author-column resizing accounts for the additional column.

Validation:

• Based on the latest upstream develop (8f8aaea), targeting develop as requested in the contribution guidelines.
• dotnet build src/SourceGit.csproj -c Debug --no-restore: passed with zero warnings or errors (using a separate output directory).
• dotnet format --verify-no-changes src/SourceGit.csproj --no-restore: passed.