Fixed issue #621 console compatibility with Eclipse 2020-03

It seems like the specification of high water mark made in MEP has been wrong for years. My guess is that the high water mark was not honored by the Eclipse platform and it has been fixed in Eclipse 2020-03.

The behavior I have been experiencing since I upgraded from Eclipse 2019-12 to 2020-03 is that the MercurialEclipse console would only hold a few lines and be cleared, as if it had been reset, when a new line was output. While working on this issue, I discovered that this was indeed the specification for the high water mark.

When the user chose not to specify a limit to the number of characters in the MEP console, the plugin arbitrarily set the limit to 1000 characters, which caused the behavior I've been experiencing. The one-line fix consisted in setting the default values of no low and high water marks, i.e. setting both values to -1. All other code changes came from applying the boy scout motto of leaving the place cleaner that it was. In particular, I've had to use Eclipse RCP 2020-03 as an API baseline, which caused a host of problems that I've documented a bit.