Merge branch 'mr/rerere-crash-fix'

When .git/rr-cache/ rerere database gets corrupted or rerere is fed to
work on a file with conflicted hunks resolved incompletely, the rerere
machinery got confused and segfaulted, which has been corrected.

* mr/rerere-crash-fix:
  rerere: fix crashes due to unmatched opening conflict markers

diff --cc rerere.c
Simple merge

diff --cc t/t4200-rerere.sh
Simple merge