For each defect in code I create separate branch. When defect is fixed I merge this branch in master, so I have history like illustrated below (we see two branches with fixes):
defect1 fix defect2 fix
a---b---c---d e---f
/ \ / \
---o---1---x---y---z---2---o---3---w---4---o---
The question is how to get diff for fix1 (between branch start (1) and branch end (2)) or fix2 (diff between (3) and (4)) at any point of time (e.g. for any closed defect in past).
Update: actual question is how to figure out SHA summs of a and d or e and f to perform next obvious diff command diff <commit> <commit>
The answer is simple:
git diff 1..d
This shows the differences between the branching point of your defect1 fix branch (i.e. 1) and it's end (i.e. d).
In order to find the start of the defect1 fix branch, use
git merge-base master defect1-fix-branch
as indicated in this answer: https://stackoverflow.com/a/2458173/520162.
This gives you 1 according to the documentation of git merge-base.
The end of the defect1 fix branch is simply identified by it's name. So, finding all differences introduced in defect1 fix, you need to do
git diff 1..defect1-fix-branch