fix(snapshot): complete gitignore respect for previously tracked files

[thdxr]

Summary

The previous fix (#22171) was incomplete. This PR adds the missing critical pieces.

What was missing in #22171

1. Missing --no-index flag in git check-ignore commands

   • Without --no-index, check-ignore returns exit code 1 for tracked files
   • This means files already in git (like a.txt) wouldn't be filtered even if added to .gitignore

2. Missing diffFull() filtering

   • When gitignore changes between two track() calls, diffFull(from, to) would still show gitignored files

The Complete Fix

In add() function:

• Uses git check-ignore --no-index to identify newly-ignored files
• Removes them from snapshot index with git rm --cached

In patch() function:

• Filters gitignored files from diff output using --no-index

In diffFull() function:

• Filters gitignored files from diff results (NEW in this PR)

Test Coverage

1. files tracked in snapshot but now gitignored are filtered out

   • Uses a.txt from bootstrap (already committed to git)
   • Verifies the fix works for files that were in git before being gitignored

2. gitignore updated between track calls filters from diff (NEW)

   • Tests that gitignore changes between two track() calls
   • Properly filters from diffFull() results

Why --no-index is critical

# Without --no-index: Returns exit code 1 for tracked files
$ git check-ignore a.txt  # a.txt is tracked → exit 1, not filtered

# With --no-index: Purely checks against patterns
$ git check-ignore --no-index a.txt  # Returns a.txt if in .gitignore

Verification

All 53 snapshot tests pass including 2 new tests.

[greptile-apps]

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