Merge branch 'lt/abbrev-auto'

Allow the default abbreviation length, which has historically been
7, to scale as the repository grows.  The logic suggests to use 12
hexdigits for the Linux kernel, and 9 to 10 for Git itself.

* lt/abbrev-auto:
abbrev: auto size the default abbreviation
abbrev: prepare for new world order
abbrev: add FALLBACK_DEFAULT_ABBREV to prepare for auto sizing

diff --cc builtin/fetch.c
Simple merge

diff --cc builtin/rev-parse.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc diff.c
Simple merge

diff --cc environment.c
Simple merge

diff --cc sha1_name.c
Simple merge

diff --cc transport.h
Simple merge