Undocumented git-diff syntax

When trying "just to see it it would work", I noticed that the following syntax appears to do what it looks like it should do.

git diff v1.7.3-rc0:Documentation/RelNotes-1.7.3.txt v1.7.3:Documentation/RelNotes/1.7.3.txt

It is a useful thing, but I could not find described in the manpage. I have not dig this part of diff yet, and maybe someone would have already a description of it (does it simply allow to diff two arbitrary blobs ?). Is it here just by some magic side-effect ? :)

Another thought: if we can already compare arbitrary blobs, maybe it would not be so hard to extend it to take arbitrary contents that are not stored as blobs yet ? (a quick hack would be to insert an object for them, but that does not look too clean...). As a use-case, I'm often found generating diffs from within custom scripts, from contents derived from a git repo, but can't use all the bells and whistles (--color, --color-words, etc), just because I have to use plain diff; that makes the output of those scripts very inconsistent with native git commands.

Would there be any objections to moving into that direction ?