hgweb: fix filediff base calculation...
hgweb: fix filediff base calculation
Previously, we were finding the most recent version of a file in a
changeset and comparing it against its first file parent. This was
wrong on three counts:
- it would show a diff in revisions where there was no change to a file
- it would show a diff when only the exec bit changed
- it would potentially compare against a much older changeset, which
could be very expensive if git-style rename detection was enabled
This compares the file in the current context with that context's
parent, which may result in an empty diff when looking at a file not
touched by the current changeset.
