environment: move object database functions into object layer

The `odb_mkstemp()` and `odb_pack_keep()` functions are quite clearly
tied to the object store, but regardless of that they are located in
"environment.c". Move them over, which also helps to get rid of
dependencies on `the_repository` in the environment subsystem.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

environment.h

@@ -200,21 +200,6 @@ extern int grafts_keep_true_parents;
 
 extern int repository_format_precious_objects;
 
-/*
- * Create a temporary file rooted in the object database directory, or
- * die on failure. The filename is taken from "pattern", which should have the
- * usual "XXXXXX" trailer, and the resulting filename is written into the
- * "template" buffer. Returns the open descriptor.
- */
-int odb_mkstemp(struct strbuf *temp_filename, const char *pattern);
-
-/*
- * Create a pack .keep file named "name" (which should generally be the output
- * of odb_pack_name). Returns a file descriptor opened for writing, or -1 on
- * error.
- */
-int odb_pack_keep(const char *name);
-
 const char *get_log_output_encoding(void);
 const char *get_commit_output_encoding(void);