scripts/getver.sh: Fix revision numbering (for Github-based repo)

scripts/getver.sh: Fix revision numbering (for Github-based repo)

Fix Chaos Calmer revision numbering. CC has been stuck at r49389 since
the final move to Github as revision number evaluation has still been
based on git-svn-id that is not found in the new original Github commits.
So the revision has been stuck at last svn commit in June.

This patch
* copies the git revision logic from master and uses v15.05.1 tag
  as the base. As the last commit with a known svn revision 49389 was
  cb4f071 with tag+135, use 49254 as the adjustment. That produces
  r49461 for the current 8a1f7c9

* removes the useless svn evaluation (similarly as in master).

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>