Implement automatic reflog pruning (90-day expiry)

[UtsavBalar1231]

Problem

Reflog grows indefinitely with no automatic pruning mechanism. Over time, reflog files become large and contain ancient history that's no longer useful.

Location

• src/reflog.rs - Reflog implementation
• No expiry logic exists

Impact

• Reflog files grow unbounded
• Wasted disk space
• Slower reflog operations as file grows
• Ancient entries rarely useful for recovery

Approach

1. Add Expiry Configuration:

pub struct Config {
    /// How long to keep reflog entries (days)
    pub reflog_expire_days: u32,  // Default: 90
    
    /// Automatically prune on operations
    pub reflog_auto_prune: bool,  // Default: true
}

2. Implement Pruning Logic:

pub fn prune_reflog(path: &Path, days: u32) -> Result<usize> {
    let cutoff = SystemTime::now() - Duration::from_secs(days as u64 * 86400);
    
    let entries = load_reflog(path)?;
    let keep: Vec<_> = entries.into_iter()
        .filter(|e| e.timestamp > cutoff)
        .collect();
    
    let pruned = entries.len() - keep.len();
    save_reflog(path, &keep)?;
    Ok(pruned)
}

3. Trigger Points:

• Automatic: After each operation (if auto_prune enabled)
• Manual: dot reflog expire command
• Scheduled: Via cron or systemd timer (user's choice)

4. Pruning Strategy:

• Keep recent entries (last 90 days by default)
• Keep entries referenced by branches/tags (never prune)
• Option to keep more entries for specific refs

5. Safety:

• Never prune entries still reachable from refs
• Warn before pruning manually
• Option for dry-run: dot reflog expire --dry-run

Git's Approach

Git has complex expiry rules:

• gc.reflogExpire: default 90 days
• gc.reflogExpireUnreachable: default 30 days
• Different expiry for reachable vs. unreachable commits

We can start simpler and add complexity if needed.

Commands to Add

# Prune expired entries
dot reflog expire

# Prune with custom expiry
dot reflog expire --expire-days=30

# Preview what would be pruned
dot reflog expire --dry-run

# Delete all reflog entries (dangerous)
dot reflog delete --all

Test Cases

• Entries older than threshold are pruned
• Recent entries are kept
• Entries referenced by branches are kept
• Manual expiry works
• Dry-run shows correct preview

Related

• Issue Ensure atomic operations in remote push/pull with proper rollback #12 (GC) - Should also prune unreachable commits
• Could combine reflog expiry with full GC operation

References

• Git reflog expiry: https://git-scm.com/docs/git-reflog#Documentation/git-reflog.txt-expire
• Git gc: https://git-scm.com/docs/git-gc