Fix common path "calculation" from logs of multiple branches.

Fix common path "calculation" from logs of multiple branches.
Need to use min instead of max for prev/cur to avoid out-of-bounds
string access. Also treat "i" as index of the last match instead of
a length because in case of a complete match of the two strings
i was off by one.

Signed-off-by: Simon Hausmann <shausman@trolltech.com>