Merge branch 'ps/odb-generalize-prepare'

The 'reprepare()' callback for object database sources has been
generalized into a 'prepare()' callback with an optional flush cache
flag, and a new 'odb_prepare()' wrapper has been introduced to allow
pre-opening object database sources.

* ps/odb-generalize-prepare:
  odb: introduce `odb_prepare()`
  odb/source: generalize `reprepare()` callback

diff --cc builtin/grep.c
Simple merge

diff --cc odb.c
Simple merge

diff --cc odb.h
Simple merge

diff --cc odb/source-files.c
Simple merge

diff --cc odb/source-inmemory.c
Simple merge

diff --cc odb/source-loose.c
Simple merge

diff --cc odb/source-packed.c
Simple merge

diff --cc odb/source-packed.h
Simple merge

diff --cc odb/source.h
Simple merge

diff --cc packfile.c
Simple merge