I have trunk and a release branch. If I fixed a bug in release branch, I definitely should integrate the fix back to trunk. However, I didn't find a command dedicated to integrate such a single change list; did I miss something?
To integrate changelist 100, for example, you'd use:
p4 merge //releasebranch/...@=100 //trunk/...
p4 resolve
p4 submit
(If you have an older Perforce server you'll have to use 'integ' instead of 'merge'.)
Note that '@=100' means the same thing as '@100,100' in this context.