Here's how I've arranged my browser to look at the official PostgreSQL git repository. I just put the hash in and go.
First, I added the LocationMorph plugin to Firefox.
Next, I added a simple rule:
Pattern: ^([0-9a-f]{40})$
Replacement: http://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=$1
Save the rule, restart Firefox, and from now on, putting a SHA1 hash in the location bar brings up the corresponding commitdiff in the PostgreSQL repository.
Enjoy!
For example: "p ......."
The way to do it, is to use "keywords" for bookmarks. I described how to do it some time ago in http://www.depesz.com/2007/09/26/easy-access-to-postgresql-documentation-from-firefox/
The cool things is that when using this method you wouldn't need whole hash, just beginning - like with normal git command line.