repo: Rename "stage" to "write" in the API

An earlier version of this API acted like git in that some objects
would be staged in a temporary directory which would be then committed
in one go by moving files around. The API doesn't match most users
expectations though, as while the stage is nice as a high-level API
it isn't really suited for low-level APIs.

While the stage was removed, the APIs were never renamed. Rename
them now so that they match expectations.

https://bugzilla.gnome.org/show_bug.cgi?id=707644

doc/ostree-sections.txt

@@ -68,14 +68,14 @@ ostree_repo_commit_transaction
 ostree_repo_commit_transaction_with_stats
 ostree_repo_abort_transaction
 ostree_repo_has_object
-ostree_repo_stage_metadata
-ostree_repo_stage_metadata_async
-ostree_repo_stage_metadata_finish
-ostree_repo_stage_metadata_trusted
-ostree_repo_stage_content
-ostree_repo_stage_content_trusted
-ostree_repo_stage_content_async
-ostree_repo_stage_content_finish
+ostree_repo_write_metadata
+ostree_repo_write_metadata_async
+ostree_repo_write_metadata_finish
+ostree_repo_write_metadata_trusted
+ostree_repo_write_content
+ostree_repo_write_content_trusted
+ostree_repo_write_content_async
+ostree_repo_write_content_finish
 ostree_repo_resolve_rev
 ostree_repo_write_ref
 ostree_repo_write_refspec
@@ -92,10 +92,10 @@ OstreeRepoCommitModifier
 ostree_repo_commit_modifier_new
 ostree_repo_commit_modifier_ref
 ostree_repo_commit_modifier_unref
-ostree_repo_stage_directory_to_mtree
-ostree_repo_stage_archive_to_mtree
-ostree_repo_stage_mtree
-ostree_repo_stage_commit
+ostree_repo_write_directory_to_mtree
+ostree_repo_write_archive_to_mtree
+ostree_repo_write_mtree
+ostree_repo_write_commit
 OstreeRepoCheckoutMode
 OstreeRepoCheckoutOverwriteMode
 ostree_repo_checkout_tree