Merge branch 'lt/default-abbrev'

* lt/default-abbrev:
  Rename core.abbrevlength back to core.abbrev
  Make the default abbrev length configurable

diff --cc Documentation/config.txt
Simple merge

diff --cc builtin/describe.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc config.c
Simple merge

diff --cc environment.c
Simple merge