evolve: fix content-divergence resolution when p1 is null (issue6201)
Before this fix, in cases like in the added test, the revision number -1 of the
parent was misinterpreted to mean the tipmost revision instead of the null
revision, causing the content-divergence resolution to fail.
syntax: re
/figures/[^/]+\.png$
^html/
\.pyc$
~$
\.swp$
\.orig$
\.rej$
^tests/.*\.err$
^tests/easy_run.sh$
^build/
^dist/
^MANIFEST$
^docs/tutorials/.*\.rst$
\.ico$
tests/\.testtimes
^docs/training/graphs/
^docs/training/html/
^docs/training/index.html
^docs/training/graphviz-images/
^docs/training/img/
^docs/training/output/
^docs/training/pandocfilters/