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