Please see here for discussion:

https://answers.atlassian.com/questions/160170/bitbucket-urls-when-browsing-repos-always-contain-hash

tl;dr

It would be nice to provide an option to create URLs with branch name instead of commit hash when browsing the repo. For the most part, when I link to file or folder in the repo, I want the link to point to the latest commit in the branch, and not the specific one.

GitHub does it nicely by generating URLs like this:
https://github.com/symfony/symfony-standard/tree/_master_/src