Merge branch 'cb/common-prefix-unification'

* cb/common-prefix-unification:
rename pathspec_prefix() to common_prefix() and move to dir.[ch]
consolidate pathspec_prefix and common_prefix
remove prefix argument from pathspec_prefix

diff --cc builtin/commit.c
Simple merge

diff --cc builtin/ls-files.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc setup.c
Simple merge