Add commitDisplayFormat option with normal/comfortable/spacious modes

[alexeRadu]

Add a git.log.commitDisplayFormat user config controlling how commits are rendered in the commits and sub-commits panels, switchable at runtime from the log menu (ctrl+l):

• normal: the existing one-line-per-commit layout
• comfortable: two lines per commit, with author and date on an indented second line, freeing the full width for the subject
• spacious: like comfortable, plus an empty graph row between commits

The extra lines are implemented as non-model items (like section headers), so commit-level navigation, search and selection are unaffected. Each inserted line starts with the graph's connector characters (rendered from the same cached pipe sets as the commit rows) so branch lanes remain continuous, including the selection highlight of the selected commit's path.

Claude-Session: https://claude.ai/code/session_01MacRoM2ci19SpyNzGnpr9S

PR Description

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc