comparing file contents in is_exact_match?

hoi :)

I created a git repository for my photo collection and then renamed
some photos (about 600).  Now git status and commit get hit by
the OOM killer.

The reason for that is that is_exact_match (in diffcore-rename.c) maps
both the source and destination file into memory and then compares them
byte for byte.  This is a little bit too much for my little machine.

When I remove the content comparation and only leave the sha1
comparision, then my renames are correctly found in a second.  But
unluckily, some other renames in the testcases are not correctly
handled any more.

So is there an easy solution?  Why is the content comparision in
is_exact_match needed?

-- 
Martin Waitz