Start to implement a built-in version of `git add --interactive`

Unlike previous conversions to C, where we started with a built-in helper, we start this conversion by adding an interception in the `run_add_interactive()` function when the new opt-in `add.interactive.useBuiltin` config knob is turned on (or the corresponding environment variable `GIT_TEST_ADD_I_USE_BUILTIN`), and calling the new internal API function `run_add_i()` that is implemented directly in libgit.a. At this point, the built-in version of `git add -i` only states that it cannot do anything yet. In subsequent patches/patch series, the `run_add_i()` function will gain more and more functionality, until it is feature complete. The whole arc of the conversion can be found in the PRs #170-175 at https://github.com/gitgitgadget/git. The "--helper approach" can unfortunately not be used here: on Windows we face the very specific problem that a `system()` call in Perl seems to close `stdin` in the parent process when the spawned process consumes even one character from `stdin`. Which prevents us from implementing the main loop in C and still trying to hand off to the Perl script. The very real downside of the approach we have to take here is that the test suite won't pass with `GIT_TEST_ADD_I_USE_BUILTIN=true` until the conversion is complete (the `--helper` approach would have let it pass, even at each of the incremental conversion steps). Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>