RFC: Switch to "rebase and merge" strategy as default

[gadomski]

Both PySTAC and stactools allow merge commits and squash merging.

PySTAC

pystac-client

Currently, pystac-client only allows squash merging:

Resolution

For consistency's sake, we should enable and default to the same strategy for all three repositories. I personally prefer rebase-and-merge, as it preserves possibly-meaningful commit history while maintaining linearity. Other thoughts?