Merge branch 'jk/abbrev-auto'

Updates the way approximate count of total objects is computed
while attempting to come up with a unique abbreviated object name,
which in turn needs to estimate how many hexdigits are necessary to
ensure uniqueness.

* jk/abbrev-auto:
find_unique_abbrev: move logic out of get_short_sha1()

diff --cc cache.h
Simple merge

diff --cc sha1_file.c
Simple merge

diff --cc sha1_name.c
Simple merge