Reorg internal odb read header and object lookup

Often `git_odb_read_header` will "fail" and have to read the
entire object into memory instead of just the header.  When this
happens, the object is loaded and then disposed of immediately,
which makes it difficult to efficiently use the header information
to decide if the object should be loaded (since attempting to do
so will often result in loading the object twice).

This commit takes the existing code and reorganizes it to have
two new functions:

- `git_odb__read_header_or_object` which acts just like the old
  read header function except that it returns the object, too, if
  it was forced to load the whole thing.  It then becomes the
  callers responsibility to free the `git_odb_object`.
- `git_object__from_odb_object` which was extracted from the old
  `git_object_lookup` and creates a subclass of `git_object` from
  an existing `git_odb_object` (separating the ODB lookup from the
  `git_object` creation).  This allows you to use the first header
  reading function efficiently without instantiating the
  `git_odb_object` twice.

There is no net change to the behavior of any of the existing
functions, but this allows internal code to tap into the ODB
lookup and object creation to be more efficient.

1 files changed, 8 insertions, 0 deletions

diff --git a/src/odb.h b/src/odb.h
index 696e12943..e9e33dde8 100644
--- a/src/odb.h
+++ b/src/odb.h
@@ -84,4 +84,12 @@ int git_odb__error_notfound(const char *message, const git_oid *oid);
  */
 int git_odb__error_ambiguous(const char *message);
 
+/*
+ * Attempt to read object header or just return whole object if it could
+ * not be read.
+ */
+int git_odb__read_header_or_object(
+	git_odb_object **out, size_t *len_p, git_otype *type_p,
+	git_odb *db, const git_oid *id);
+
 #endif