Add --output-start-date and --output-stop-date for segment rendering

[ktamas77]

Summary

This PR adds two new command-line options that enable segment rendering - the ability to render only a specific date range while still simulating the full repository history.

New Options

• --output-start-date "YYYY-MM-DD hh:mm:ss +tz" - Start outputting frames from this date
• --output-stop-date "YYYY-MM-DD hh:mm:ss +tz" - Stop outputting frames after this date

Key Difference from Existing Options

The existing --start-date and --stop-date options filter log entries (commits), which means:

• Files that existed before --start-date won't appear
• The visualization starts from scratch at the specified date

The new --output-start-date and --output-stop-date options simulate the full history but only output frames within the specified range:

• All files and contributors are in their correct positions
• The tree structure reflects the complete repository state at that point in time
• Only the rendering output is filtered, not the simulation

Motivation / Use Case

When rendering visualizations for large repositories with years of history, it's often desirable to produce weekly or monthly video segments rather than one massive video.

Current approach (inefficient):

# Render January - processes ALL history, outputs ALL frames
gource --start-date "2024-01-01" --stop-date "2024-01-31" -o jan.ppm

# Render February - starts from scratch, files missing
gource --start-date "2024-02-01" --stop-date "2024-02-28" -o feb.ppm

With this PR (efficient segment rendering):

# Render January segment - full history simulated, only Jan frames output
gource --output-start-date "2024-01-01" --output-stop-date "2024-01-31" -o jan.ppm

# Render February segment - full history simulated, only Feb frames output  
gource --output-start-date "2024-02-01" --output-stop-date "2024-02-28" -o feb.ppm

Each segment shows the repository in its correct state, and segments can be concatenated into a seamless video.

Implementation

Files Modified

1. src/gource_settings.h - Added new member variables:

   • output_start_timestamp, output_stop_timestamp (time_t)
   • output_start_date, output_stop_date (std::string)

2. src/gource_settings.cpp - Added:

   • Argument type definitions
   • Default value initialization
   • Parameter parsing (reusing existing parseDateTime function)
   • Help text

3. src/gource.cpp - Modified update() to check date range before frameExporter->dump():

   • Skip frames before output_start_timestamp
   • Skip frames after output_stop_timestamp and set stop_position_reached

4. README.md - Added documentation for both new options

Design Decisions

• Follows the same date parsing format as existing --start-date/--stop-date
• Uses stop_position_reached flag when passing output_stop_date to properly exit when using --stop-at-end
• Minimal code changes - reuses existing infrastructure
• Backward compatible - no changes to existing behavior

Testing

Tested locally on macOS with a sample repository:

./gource . --output-start-date "2011-10-01" --output-stop-date "2011-10-31" \
  --output-framerate 60 --seconds-per-day 0.1 --stop-at-end -o - | \
  ffmpeg -y -r 60 -f image2pipe -vcodec ppm -i - -vcodec libx264 output.mp4

Successfully produced a video showing only October 2011 activity with the full repository tree structure intact.