$ hg init repo
  $ cd repo

  $ echo file1version1 > file1
  $ echo file2version1 > file2
  $ echo file3version1 > file3
  $ hg commit -Aqm "base"
  $ echo file1version2 > file1
  $ hg commit -qm "file1 change1"
  $ echo file2version2 > file2
  $ hg commit -qm "unrelated change to file2"
  $ echo file1version3 > file1
  $ hg commit -qm "file1 change2"
  $ echo file3verison2 > file3

Normal diff shows the unrelated change in the intervening commit.
  $ hg diff -r 1 -r 3 --nodates
  diff -r b586868a82b9 -r 362c080b3cff file1
  --- a/file1
  +++ b/file1
  @@ -1,1 +1,1 @@
  -file1version2
  +file1version3
  diff -r b586868a82b9 -r 362c080b3cff file2
  --- a/file2
  +++ b/file2
  @@ -1,1 +1,1 @@
  -file2version1
  +file2version2

With --only-files-in-revs, that is excluded.
  $ hg diff -r 1 -r 3 --nodates --only-files-in-revs
  diff -r b586868a82b9 -r 362c080b3cff file1
  --- a/file1
  +++ b/file1
  @@ -1,1 +1,1 @@
  -file1version2
  +file1version3

Similarly, with a single rev, only consider files modified in that rev and the working copy.
  $ hg diff -r 1 --nodates --only-files-in-revs
  diff -r b586868a82b9 file1
  --- a/file1
  +++ b/file1
  @@ -1,1 +1,1 @@
  -file1version2
  +file1version3
  diff -r b586868a82b9 file3
  --- a/file3
  +++ b/file3
  @@ -1,1 +1,1 @@
  -file3version1
  +file3verison2