splitUnifiedDiff used bytes.Buffer to accumulate each file's diff,
then stored buf.Bytes() into the result map and called buf.Reset()
to start the next file. bytes.Buffer.Bytes() returns the buffer's
internal backing slice; Reset() resets length to 0 but reuses the
same backing array. As a result, every map entry aliased the same
storage, so all files ended up showing the LAST file's diff content.
Fix: copy the bytes into a fresh slice before storing in the map.
Adds TestPRFilesDiffPerFileIsolation as a regression test that
asserts each file entry contains its OWN diff --git header and
none of the other files' headers. Verified failing on the prior
code, passing after the fix.
Closes#25
9013c8ff9c