Add colour support in rebase and merge tree diff stats output.

Add colour support in rebase and merge tree diff stats output.

The rebase and merge commands used diff-tree to display the summary stats of
what files had changed from the operation. diff-tree does not read the
diff ui configuration options, so the diff.color setting was not used.

Have rebase and merge call diff rather than diff-tree, which does read the
diff ui options.

Signed-off-by: James Bowes <redacted>
Signed-off-by: Junio C Hamano <redacted>