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I really (really) don't want to turn this into a 'my SCM is better than yours' discussion but the problems mentioned above are (one of) the reasons why I've switched from SVN to Git. Git does require a bit of reading and a different way of thinking about version control but this (small) investment in time/effort really pays of in the end.

One possible way to use a vendor branch like approach with Drupal in Git:

http://versioncontrolblog.com/2007/08/02/upgrading-drupal-52-with-git/

I've been using SVN for quite some time and was rather reluctant to learn about Git (time is scarce :D) but in the end it was really worth it, especially when working with other developers or when using external components like Drupal.