Add Git Log Overview mode

[damiensawyer]

Hi Jesse. I note and respect your comment that "This project does not accept pull requests".

I submit this anyway in the hope that might get through your gate. I personally think it's a useful feature.

PR Description

When a branch's history is full of individual, non-squashed commits, the commits panel gets stretched out vertically to the point that it's hard to see the bigger picture — when a branch was merged in, what merged where, and how the history is structured. This adds an overview mode (toggled with ~, also available from the log options menu) that condenses the list down to just the commits that carry that structural information, hiding everything else:

• merge commits
• commits pointed to by a local branch, remote branch, tag, or HEAD
• rebase todo entries (so mid-rebase views still work)

The result is a vertically condensed view where you can see at a glance when and where branches were merged into the one you're looking at, without scrolling past dozens of intermediate commits to piece it together.

The git graph keeps exactly the same shape it has in the full log — lane positions and merge join lines don't shift when rows are hidden, since the graph is computed from the full commit list and only the relevant rows are rendered.

Selection handling: if the selected commit gets hidden when the mode is turned on, the selection jumps to the nearest visible commit above it and scrolls into view. Turning the mode back off returns to the original commit, as long as the selection wasn't moved while in overview mode.

Commit limit: overview mode disables the commit-limit/load-more heuristic, since the condensed list can be much shorter than the threshold that heuristic relies on.

UI feedback: while active, the mode shows in the information panel with a reset button, and the commits view's title/tab gets an (overview) suffix, so hidden commits are never mistaken for missing history.

Guarded commands: actions that depend on the selection's position in the real commit list (squash, fixup, edit, move up/down, reword, drop, amend, etc.) prompt to exit overview mode first — the same pattern used by filter mode — since "the commit below" on screen isn't the commit below in the real history while rows are hidden.

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (go generate ./...)
• Code has been formatted (gofumpt, golangci-lint clean)
• Tests have been added/updated (toggle_overview_mode, toggle_overview_mode_scrolls_selection_into_view integration tests; unit tests for filterCommitsForOverview and GetCommitListDisplayStrings with displayIndices)
• Text is internationalised (all new strings added to pkg/i18n/english.go with named placeholders)
• New UserConfig entry (Keybinding.Commits.ToggleOverviewMode) hot-reloads like other action keybindings — no special handling needed
• Docs updated (docs-master/Config.md, schema-master/config.json, keybinding cheatsheets — all regenerated)
• Read through the diff for mistakes