Correctly handle the absence of one version of in a merge

Closes #1217.
magit-wash-log-line: fix #1218

All log washing except bisect log goes through `magit-wash-log',
which let-binds `magit-log-line' to 0, even when the log isn't
being truncated.  Only increase `magit-log-count' when the mode
derives (is) `magit-log-line'.  In the bisect case we have to
avoid `cl-incf'ing `magit-log-count's global value which also
happens to be `nil'.
Merge pull request #1219 from vanicat/t/ediff-merge

Correctly handle the absence of one version of in a merge