config: free resources of `struct config_store_data`

config: free resources of `struct config_store_data`

Commit fee8572c6d (config: avoid using the global variable `store`,
2018-04-09) dropped the staticness of a certain struct, instead letting
the users create an instance on the stack and pass around a pointer.

We do not free all the memory that the struct tracks. When the struct
was static, the memory would always be reachable. Now that we keep the
struct on the stack, though, as soon as we return, it goes out of scope
and we leak the memory it points to. In particular, we leak the memory
pointed to by the `parsed` and `seen` fields.

Introduce and use a helper function `config_store_data_clear()` to plug
these leaks. The memory tracked here is config parser events. Once the
users (`git_config_set_multivar_in_file_gently()` and
`git_config_copy_or_rename_section_in_file()` at the moment) are done,
no-one should be holding on to a pointer into this memory.

There are two more members of the struct that are candidates for freeing
in this new function (`key` and `value_regex`). Those are actually
already being taken care of. The next couple of patches will move their
freeing into the function we are adding here.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>