Boar's charge doesn't interrupt, it just roots the taret as targets casting a heal are not moving anyway, this will not stop the heal. However, the boar does push the spell back as it hits with charge and gore.
Also, go ahead and keep growl on a boar/cat at 29, because you will NOT be getting dash pre-level-32 (or prowl for that matter). As such, a boar is about the only pet that will actually get to the target quick, although all pets have a base +15% run speed.
That said, in PvP you want mobility, survivability, and burst damage. Pets don't need survivability, as no player in his right mind would attack the pet unless the pet is the only thing not currently crowd controlled. So, you're down to mobility and burst damage (or poison).
Some useful pets are:
-Boar...charge+gore provides some nice burst damage, and like I said earlier - boars are the most mobile pets pre-32.
-Cat/Raptor/Ravager...all of these pets have a focus dump and very high damage output. Ravager is better (slightly) but they're all three +10% DPS pets.
-Birds...Carrion Birds, Owls, and Bats provide a debuff to lower enemy meleer's damage. In a place with a lot of warriors, rogues, and hunters, this could be useful. Screech also deals some damage, so it's not wasted focus by any means, although there are probably better pets out there.
-Wind Serpent...Very high burst damage output, and the abilitiy to damage targets while rooted.
-Scorpid...while not having a very high damage output, scorpids are great for their poison. Poisons can cause a target who is bandaging to stop bandaging, and can prevent rogues/druids/elves from going stealth.
-Serpent...same benefits of the scorpid, only from a range.
At level 26-29, a boar (Bellygrub or a boar from RFK), a cat (from several locations), a raptor (from wetlands), bat (from RFK), or wind serpent (from 1k) would be fairly easy to get and level with you. For the others, you'd have to start over or do a LOT of grinding on bottom-level green mobs.