diff hunk old_count and new_count attributes are always 0 in XML output

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Description

In tools/diff/diff.go, the DiffHunk struct has OldCount int and NewCount int fields but they are never assigned during hunk construction. Only OldStart and NewStart are set.

Impact

XML consumers that rely on old_count/new_count to reconstruct or display diffs receive incorrect zero values. The unified diff format requires these (@@ -10,5 +10,7 @@).

Steps to Reproduce

aict diff file1.go file2.go --xml

Observe: <hunk old_start="10" old_count="0" new_start="10" new_count="0">

Expected Behavior

old_count = number of removed + context lines in the hunk
new_count = number of added + context lines in the hunk

Fix

After building each hunk's Lines slice, compute counts before appending to hunks:

hunk.OldCount = countLines(hunk.Lines, "removed") + countLines(hunk.Lines, "context")
hunk.NewCount = countLines(hunk.Lines, "added") + countLines(hunk.Lines, "context")

Related

tools/diff/diff.go computeDiff function