svnmerge.py is a tool for automatic branch management in Subversion. (NB: See Subversion 1.5 Release Notes for merge tracking features added in July 2008.) It allows branch maintainers to merge changes from and to their branch very easily, and automatically records which changes were already merged. This allows displaying an always updated list of changes yet to be merged, and totally prevents merge mistakes (such as merging the same change twice).
You can think of svnmerge as the equivalent of yellow sticky notes on a Subversion branch that record which revisions have been merged in already from one or more other branches, plus some useful functionality for viewing, managing and updating that information.
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