Documentation of commit-db file format

[mernst]

https://homes.cs.washington.edu/~rjust/defects4j/html_doc/Vcs.html says:

The commit-db (csv) file has the structure: bug_id,revision_buggy,revision_fixed.

However, the commit-db file contains additional columns. There may need to be changes elsewhere in the documentation to reflect the additional columns.

In addition, there should be a link from somewhere in the Defects4J documentation to https://homes.cs.washington.edu/~rjust/defects4j/html_doc/Vcs.html, with anchor text like "commit-db file format".

[Greg4cr]

There are documentation updates in #312 that (at least partially) address this. See: https://github.com/rjust/defects4j/pull/312/files#diff-e9a5de0fbb65e6be8727fbc244957048