Merge branch 'pw/adopt-c99-bool-officially'

Declare weather-balloon we raised for "bool" type 18 months ago a
success and officially allow using the type in our codebase.

* pw/adopt-c99-bool-officially:
  strbuf: convert predicates to return bool
  git-compat-util: convert string predicates to return bool
  CodingGuidelines: allow the use of bool

diff --cc Documentation/CodingGuidelines
Simple merge

diff --cc git-compat-util.h
Simple merge