Publish to gh-pages using amend commit

[ThierryO]

Description

Storing the history of the rendered site is not always required. Allowing to amend the latest commit in the gh-pages branch keeps the repo size smaller.

Replacing

quarto-cli/src/publish/gh-pages/gh-pages.ts

Line 207 in af454e8

["commit", "--allow-empty", "-m", "Built site for gh-pages"],

with

      ["commit", "--amend", "--allow-empty", "-m", "Built site for gh-pages"],

would make amending the commit the default setting.