fix: pass non-unified-diff input through to stdout (closes #115)

[Booyaka101]

Summary

When diffnav is configured as pager.diff (per the README), git pipes the output of every git diff invocation through diffnav, including non-unified-diff variants like git diff --stat, --shortstat, --name-only, and --name-status. Those produce summary text with no diff --git markers, which causes two latent bugs:

1. Panic. The bluekeyes/go-gitdiff parser returns zero files, the TUI tries to render a scrollbar over an empty viewport, and the program panics with runtime error: integer divide by zero in RenderScrollbar (pkg/ui/common/scrollbar.go:18).
2. Escape-sequence leak. Before the panic, bubbletea's terminal-capability handshake leaks [?2026\$p[?2027\$p query bytes to the user's stdout — the symptom in the bug report.

Fix

Detect non-unified-diff input early in cmd/root.go (after stdin is fully read, before opening the TTY) and pass it through verbatim. The caller sees the same output they would have without diffnav as the configured pager — same behaviour as cat/the implicit pager-fallthrough that other TUI pagers use for non-diff input.

Detection is a strings.Contains(input, "diff --git ") check, extracted into a small isUnifiedDiff helper for unit testing. Both git diff and git show always emit at least one diff --git header in the unified form, so the check matches the TUI's actual input contract.

Test plan

• 8 new unit tests in cmd/input_test.go covering:
  • git diff (unified)
  • git show (preamble + unified)
  • git diff --stat
  • git diff --shortstat
  • git diff --name-only
  • git diff --name-status
  • git log (no patch)
  • empty input
• Empirically reproduced on Windows with the canonical --stat output:
  • Pre-fix: [?2026\$p[?2027\$p bytes leak, then runtime error: integer divide by zero panic.
  • Post-fix: stat output is printed verbatim and diffnav exits 0.
• Real unified-diff input still routes to the TUI (verified by piping gh_dash_pr.diff test fixture into the post-fix binary — TUI starts, hits the same pre-existing panic the upstream test environment hits, confirming the input is reaching the renderer).

Notes

• The two pre-existing pkg/ui/panes/filetree test failures on main (TestNoLastPath, TestCollapseTree) are unrelated lipgloss-styling differences and not touched by this PR.
• This is a strict superset: any input that previously rendered now still renders; only inputs the renderer would have crashed on now exit gracefully.
• Closes Causes git diff --stat to not work #115. Should also fix the git log symptom from Does not work with git log #28 (re-emerged after the recent v2 rewrite, since git log without -p produces no diff --git line and would hit the same path).

AI usage disclosure (per AI_POLICY.md)

Tool: Claude Code (Opus 4.7).
Extent: pattern analysis (root cause for the panic + escape leak), code drafting for the isUnifiedDiff helper and the early-return path, and the test cases. I reviewed every change manually, ran go build, go test ./cmd/..., and reproduced the pre-/post-fix behaviour locally on Windows before pushing. Happy to walk through any line on request.

[dlvhdr]

thank you!

[dlvhdr]

Let's remove this explanation

[Booyaka101]

Done — removed in 0b8101b. isUnifiedDiff's godoc already covers the intent.

[dlvhdr]

thank you!