Fix a bug in smerge where files with long lines would not merge correctly in diff3-mode.
Fix a bug in smerge where files with long lines would not
merge correctly in diff3-mode.

bk: 41b0c5321ThG_Dw2PuigBHyKO67QMg
bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper,bitkeeper-scm/bitkeeper