A brief summary of the approach:
1. Discover bug.
2. Identify commit that introduced bug (git blame is invaluable here.) Can use git tag to keep track of the commit, if desired.
3. Create a bugfix branch rooted at the commit identified in step (2): git checkout -b my-bugfix-branch
4. hack hack hack…bug fixed
5. Identify branches affected by the bug: git branch —contains
6. For each branch identified in (5), merge in the bugfix branch
A wonderful deck from the always helpful Addy Osmani. 236 slides of awesome.