Merge branch 'mb/config-document-include'

Add missing documentation for "include" and "includeIf" features in
"git config" file format, which incidentally teaches the command
line completion to include them in its offerings.

* mb/config-document-include:
  config.txt: document include, includeIf

diff --cc Documentation/config.txt
Simple merge

diff --cc t/t9902-completion.sh
Simple merge