object-name: free leaking object contexts

While it is documented in `struct object_context::path` that this
variable needs to be released by the caller, this fact is rather easy to
miss given that we do not ever provide a function to release the object
context. And of course, while some callers dutifully release the path,
many others don't.

Introduce a new `object_context_release()` function that releases the
path. Convert callsites that used to free the path to use that new
function and add missing calls to callsites that were leaking memory.
Refactor those callsites as required to have a single return path, only.

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

object-name.h

@@ -22,6 +22,8 @@ struct object_context {
 	char *path;
 };
 
+void object_context_release(struct object_context *ctx);
+
 /*
  * Return an abbreviated sha1 unique within this repository's object database.
  * The result will be at least `len` characters long, and will be NUL