You are looking for --diff-filter=M
to show only files *M*odified between the two branches.
From man git-diff
--diff-filter=[ACDMRTUXB*]
Select only files that are
A
AddedC
CopiedD
DeletedM
ModifiedR
RenamedT
have their type (mode) changedU
UnmergedX
UnknownB
have had their pairing Broken*
All-or-noneAny combination of the filter characters may be used.
When * (All-or-none) is added to the combination, all paths are
selected if there is any file that matches other criteria in the
comparison; if there is no file that matches other criteria, nothing
is selected.