Index / glossary adjustments for Git & GUC

Howdy folks,

Two small "clean up" patches. These are against HEAD, though could
likely be backpatched to v18 if one were inclined.

The first patch removes the term "Git" from the acronyms section and
instead creates an index entry that points to our source repository
information.

For the second, ISTM we use the term GUC to refer to parameters pretty
liberally, but the term is not that easy to find in the docs (and
website search gives what is probably the best answer as result #7 of
8). This creates an index entry for the term, as well as a glossary
entry which explains how it is commonly used.

Robert Treat
https://xzilla.net