How to force a diff format for TextMate?
TextMate can use either the file extension or the first line of a file to divine what filetype it is.

For my diff bundle (I think it's the original), this is:

firstLineMatch = '(?x)^
        (===\ modified\ file
        |==== \s* // .+ \s - \s .+ \s+ ====
        |Index:\ 
        |---\ [^%]
        |\*\*\*.*\d{4}\s*$
        |\d+(,\d+)* (a|d|c) \d+(,\d+)* $
        |diff\ --git\ 
        )
    ';


I don't have mercurial, but it appears as though its diff's first line may look something like diff -r 2278160e78d4 hello.c.  As such, you can add another line to that big OR switch:

        ...
        |diff\ --git\ 
        |diff\ -r\ 
        )
    ';


Note that the final character in the line is a space.

As another alternative, it appears as though you may be able to define your alias as alias hgd='hg diff --git|mate' to use the same format as git, which is already recognized by TextMate.