This adds a new VCS update implementation to the cmCTestVC hierarchy and
removes it from cmCTestUpdateHandler. The new implementation has the
following advantages:
- Factorized implementation instead of monolithic function
- Logs vcs tool output as it is parsed (less memory, inline messages)
- Uses one global svn log instead of one log per file
- Reports changes on cvs branches (instead of latest trunk change)
- Generates simpler Update.xml (only one Directory element per dir)
Shared components of the new implementation appear in cmCTestVC and may
be re-used by subclasses for other VCS tools in the future.
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