How to diff 2 file revisions with gitk

Hi there:

I asked a few days ago whether I could easily diff 2 file revisions with the mouse in gitk, but I got no reply yet, see here:

   How to diff two file revisions with the mouse (with gitk)
   https://groups.google.com/forum/#!topic/git-users/9znsQsTB0dE

I am hoping that it was the wrong mailing list, and this one the right one. 8-)

Here is the full question text again:

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I would like to start gitk, select with the mouse 2 
revisions of some file and then compare them, hopefully with an external
 diff tool, very much like I am used to with WinCVS.

The closest I
 got is to start gitk with a filename as an argument, in order to 
restrict the log to that one file. Then I right-click on a commit (a 
file revision) and choose "Mark this commit". However, if I right-click 
on another commit and choose "Compare with marked commit", I get a full 
commit diff with all files, and not just the file I specified on the 
command-line arguments.

Selecting a filename in the "Tree" view and choosing "Highlight this only", as I found on the Internet, does not seem to help.

I have git 1.7.9 (on Cygwin). Can someone help?

By the way, it would be nice if gitk could launch the external diff tool from the "Compare with marked commit" option too.

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Thanks in advance,
   rdiez