Contributing to Pro Git (CONTRIBUTING.md): Conflates issues and issue reports

[Chealer]

CONTRIBUTING.md is confused/confusing, calling issue reports "issues" a few times, in the Signaling an Issue and Small Corrections sections.

The first occurrence is:

Search for similar issues, before creating a new issue.

I suggest rephrasing this as:

Before reporting an issue, search the tracker to try finding a pre-existing report. Avoid creating a duplicate ticket.