When users configure a TUI/interactive diff pager (e.g., diffnav, tig)
via `git config pager.diff`, fzf preview panes show blank output because
TUI pagers require a TTY that fzf previews don't provide.
Add FORGIT_PREVIEW_PAGER env var that overrides the diff pager only in
fzf preview context. Preview detection relies on a FORGIT_IN_PREVIEW
marker set by a new `_forgit_preview` wrapper, so the override applies
only to preview commands and leaves fullscreen behavior unchanged.
Fixes#491
