In the 1960's was when you wrote software by punching cards that someone else fed in and where it had to work the first time. Every time. That kind of discipline is sorely needed by the original question submitter.
The whole haphazard development model described in the question is absurd. First of all, what kind of single bug requires rifling through back end databases, business rules, web services and multiple front ends? That's not a bug in the software, that's a bug in the pre-design definitions phase. That is not a bug. Seriously... you can't just accept all the premises in the question without thought. That kind of change only happens when someone is is calling "the customer wants this feature changed" or "we misunderstood what the customer needed" a bug, which is wrong on its face.
Secondly, multiple people making changes of that scope simultaneously is just wrong, whatever the cause. Distributed revision control systems were made able to handle multiple simultaneous branches in order to break bottlenecks on people working on different areas of a common source file. They were designed to accommodate merges that had occasional and minor overlaps. What is described here is a completely inappropriate use of that kind of environment. So to answer the question directly, when asked what tools can help, the answer is no tools can help you. The process is wrong. You are far better off reverting to a revision control system that enforces a single checkout of a source file if this is what is going on. Better yet, correct your development strategy.
This can't be emphasized strongly or often enough. Code ownership is a good step forward in this scenario, but the only real fix for these problems is to completely refactor the way change is managed in this project. You wouldn't be wrong to Gantt chart these changes with their subsystem impacts so they can be scheduled on a non-interference basis. Better yet, if you are having to make multiple back-end through to UI changes, you need to go through a whole scope identification phase again.
Your change system is hopelessly broken. Fix that, then the correct use of existing tools to assist you will become readily apparent.