Merge branch 'jk/error-const-return'

Help compilers' flow analysis by making it more explicit that
error() always returns -1, to reduce false "variable used
uninitialized" warnings.  Looks somewhat ugly but not too much.

* jk/error-const-return:
  silence some -Wuninitialized false positives
  make error()'s constant return value more visible

diff --cc cache.h
Simple merge

diff --cc config.c
Simple merge

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