I believe this changed when support for Git was added: the generated URLs that reference changesets use the full 40 character hexadecimal node ID.
I understand that it's nice to provide the full IDs: that way you don't need to implement logic to handle collisions any better than the underlying tools, i.e., you don't need to handle them on the assumption that SHA-1 is a secure hash function.
The long links are cumbersome: instead of a handy short node ID (12 characters for Mercurial) that will fit into most search boxes without clipping, I'm giving a 40 character string. That string is way to big for me to handle as anything but an opaque blob of bytes.
With a short node ID I can glance at it in an email and quickly see if it looks like it's the same as the one shown in my browsers address bar. I can copy a small node ID by hand in full — with a long ID I need to decide when to give up and when my prefix is long enough. I can paste a short ID into most tools without it overflowing the input field. Overall short IDs are more convenient and handy.
The long links are almost always longer than what I'm comfortable pasting into text. The fixed text (
https://bitbucket.org/.../.../commits/) take up 30 characters, so with 40 characters for the node ID we're already at 70 and then you need to add the username and repository.
The result is a 80+ character link which flows very badly in a text file or an email. I end up with lines longer than 80 characters which means that my diffs display badly in a standard width terminal.
The long links are mostly unnecessary. There are no collisions in the 275k changesets in the OpenOffice repository among the first 10 hexadecimal characters — there is a single collision among the first 9 characters.
This means that short links will work well in practice for almost all repositories.
You should actually handle collisions anyway: today I can shorten the Bitbucket URLs and they still work:
However, I get a "404 Not Found" if the prefix becomes ambiguous:
That ought to be handled better: present the user with a list of matching changesets