Show dynamic commit and size values during growth data collection

[nicklegan]

Problem

The progress spinner in the HISTORIC & ESTIMATED GROWTH table showed 0 for commits and placeholder dots (...) for delta and percentage columns while data was being collected. On large repositories like the Linux kernel, this made it appear stuck for extended periods.

Solution

Track the previous year's cumulative statistics alongside the current statistics so the progress display can compute and show meaningful real-time values as each year completes:

• Commits: cumulative count and year-over-year delta (+N)
• Object size: cumulative uncompressed size and delta
• On-disk size: cumulative compressed size and delta
• Level of concern: updated during progress based on the currently collected values

The % columns remain ... during progress collection because interim totals are not final and would produce unstable values that do not match the completed report.

Values still fall back to ... when genuinely zero (for example, before any data is collected).

Changes

• pkg/progress/progress.go: add PreviousStatistics, helper functions for delta formatting, update progress rendering to show live absolute values, deltas, and level of concern, and keep % as placeholders during collection
• main.go: track beforePrevious and pass it to StartProgress; update PreviousStatistics after each year completes
• pkg/progress/progress_test.go: update StartProgress calls for the new signature

[steffen]

@nicklegan This works great! 💖

Regarding the % column: The % column shows the percentage of the delta in that year relatively to the current total value of the repository. So as git-metrics goes through the years, that percentage value changes (as it doesn't have the total yet) and the percentages shown during the progress will not match the ones that are shown once the complete table is calculated and done. E.g. if you have a repository with 4 years history and each year as 25,000 commits, it shows 100% when it has calculated the first year, then it shows 50% and then 25%.
So I think for now we remove updating the percentage in the progress line and only update the absolute numbers and deltas (Δ) and level of concern (○). What do you think?

[Copilot]

Pull request overview

Improves the CLI’s growth-data progress spinner so it shows meaningful commit and size values while yearly growth statistics are being collected, reducing the “stuck at 0/…” perception on large repositories.

Changes:

• Extend progress state to carry both current and previous cumulative growth statistics.
• Compute and display commit and size deltas in the progress line during collection.
• Update StartProgress call sites to pass the additional “previous statistics” argument.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
pkg/progress/progress.go	Adds PreviousStatistics to progress state and formats deltas for commits/object size/on-disk size in the spinner output.
main.go	Tracks an additional “before previous” cumulative statistics value and wires it into progress initialization/updates.
pkg/progress/progress_test.go	Updates unit tests to match the new StartProgress function signature.

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