bitbucket: Decouple BITBUCKET_FORK_BODY between versions.
bitbucket: Decouple BITBUCKET_FORK_BODY between versions.

We were trying to share the same format string between
the two different versions of bitbucket, but this only
creates confusion, as the two versions are only close
enough to be confusing.

The format string might be the same, but the semantics
are different, as well as the eventual outputs.

For example, the {username} piece here is simple in version
2, but in version 3 we append a url to the user's name.

showell/zulip,hackerkid/zulip,eeshangarg/zulip,kou/zulip,showell/zulip,kou/zulip,showell/zulip,zulip/zulip,punchagan/zulip,andersk/zulip,punchagan/zulip,andersk/zulip,rht/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,andersk/zulip,zulip/zulip,showell/zulip,zulip/zulip,hackerkid/zulip,showell/zulip,punchagan/zulip,kou/zulip,andersk/zulip,andersk/zulip,punchagan/zulip,hackerkid/zulip,zulip/zulip,eeshangarg/zulip,hackerkid/zulip,hackerkid/zulip,rht/zulip,zulip/zulip,kou/zulip,zulip/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,eeshangarg/zulip,showell/zulip,hackerkid/zulip,rht/zulip,showell/zulip,punchagan/zulip