Split-commit captured the staged diff with `git diff --cached --binary`,
whose stdout is hard-capped at GIT_COMMAND_OUTPUT_LIMIT_BYTES (8 MiB) by
readCappedText. A single large binary (base85-encoded inline) crossed the
cap; the capture was truncated silently, so files sorting after the binary
were absent from the parsed diff and stage.hunks threw a misleading
`No diff found for <path>` naming an innocent file.
Surface truncation as GitCommandResult.truncated, add a requireComplete
diff option that throws the new GitOutputTruncatedError instead of
returning a silently truncated diff, and have runSplitCommit request a
complete diff and abort with a clear message pointing at the real cause.
Fixes#8897
roboomp