Merge branch 'ew/decline-core-abbrev'

The configuration variable 'core.abbrev' can be set to 'no' to
force no abbreviation regardless of the hash algorithm.

* ew/decline-core-abbrev:
  core.abbrev=no disables abbreviations

diff --cc Documentation/config/core.txt
Simple merge

diff --cc config.c
Simple merge

diff --cc t/t3200-branch.sh
Simple merge