aportgen: Allow retaining current branch when forking Alpine

Often I find myself confused and subsequently frustrated by the behaviour of --fork-alpine as usually I apply whatever changes I want to the aports package before forking. This is due to that in the proper aports repo I can leverage the full power of git and apply patches or revert commits, whereas after the package has been forked to pmaports the context git needs to do this is lost.

To fix that, this commit introduces --fork-alpine-retain-branch which works the same way as --fork-alpine except it doesn't change the aports branch to match the current channel.