Fix clash with named anchor ID

[rahrah]

Rename the anchor ID [[_default_branch]] to [[_new_default_branch]], updating the reference to it.

Fixes asciidoctor build warnings due to pre-existing [[_default_branch]] anchor ID of the same name.

• [x] I provide my work under the project license.
• [x] I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

• Change the anchor ID name and/or reference in:

  book/01-introduction/sections/first-time-setup.asc 
  book/02-git-basics/sections/recording-changes.asc 

  This clashed with:

  book/06-github/sections/3-maintaining.asc

Context

commit 2af8f742c69c8cfac44ce56e1b51701948be93bc

added a reference [[_default_branch]].

commit 1b64de1eb4023847fe371289ec2ca58ede6902be

adds a links to that reference.

However, the reference already existed in:

book/06-github/sections/3-maintaining.asc

created in 2014 with:

commit bf7ce71d803a33cf98a347527c9369221040a01c

Rename the new reference to [[_new_default_branch]] and update the link to it.

Fixes asciidoctor build warnings (and possible future broken links).

asciidoctor: WARNING: book/06-github/sections/3-maintaining.asc: line 363: id assigned to block already in use: _default_branch