commit --interactive: make it work with the built-in `add -i`

The built-in `git add -i` machinery obviously has its `the_repository` structure initialized at the point where `cmd_commit()` calls it, and therefore does not look at the environment variable `GIT_INDEX_FILE`. But when being called from `commit --interactive`, it has to, because the index was already locked in that case, and we want to ask the interactive add machinery to work on the `index.lock` file instead of the `index` file. Technically, we could teach `run_add_i()`, or for that matter `run_add_p()`, to look specifically at that environment variable, but the entire idea of passing in a parameter of type `struct repository *` is to allow working on multiple repositories (and their index files) independently. So let's instead override the `index_file` field of that structure temporarily. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>