Merge branch 'add-i-in-c-status-and-help'

The first part of the long journey to a fully built-in `git add -i`.

It reflects the part that was submitted a couple of times (see
gitgitgadget#103) during the Outreachy
project by Slavica Đukić that continued the journey based on an initial
patch series by Daniel Ferreira.

This part only implements the `status` and the `help` part, like
Slavica's last iteration did, in the interest of making the review
remotely more reviewable. I fear that this attempt of making it a bit
more reviewable is pretty futile, as so many things changed. So I will
ask the reviewers for forgiveness: please be kind, and give this sort of
a fresh review.

I threw in a couple of major changes on top of that iteration, though:

- The original plan was to add a helper (`git add--helper`) that takes
  over more and more responsibility from the Perl script over the course
  of the conversion.

  This plan is no longer in effect, as I encountered a serious problem
  with that: the MSYS2 runtime used by the Perl interpreter which Git
  for Windows employs to run `git add -i` has a curious bug (that is
  safely outside the purview of this here patch series) where it fails
  to read from standard input after it spawned a non-MSYS2 program that
  reads from standard input. To keep my `git add -i` in a working state,
  I therefore adopted a different strategy:

  Just like `git difftool` was converted by starting with a built-in
  that did nothing but handing off to the scripted version, guarded by
  the (opt-in) `difftool.useBuiltin` config setting, I start this patch
  series by a built-in `add -i` that does nothing else but state that it
  is not implemented yet, guarded by the (opt-in)
  `add.interactive.useBuiltin` config setting.

  In contrast to the `git difftool` situation, it is quite a bit easier
  here, as we do not even have to rename the script to
  `git-legacy-add--interactive.perl`: the `add--interactive` command is
  an implementation detail that users are not even supposed to know
  about. Therefore, we can implement that road fork between the built-in
  and the scripted version in `builtin/add.c`, i.e. in the user-facing
  `git add` command.

  This will also naturally help with the transition to a fully built-in
  `git add -i`/`git add -p`, as we saw with the built-in `git rebase`
  how important it is for end users to have an escape hatch (and for
  that reason, tried our best to provide the same with the built-in `git
  stash`).

- The `help` command was actually not hooked up in `git add -i`, but was
  only available as a special option of the `git add--helper` command.
  As that command no longer exists, I kind of *had* to implement
  some way to let the built-in `git add -i` show the help text.

- The main loop of `git add -i` (i.e. the thing that lets you choose
  `status` or `help`) is now implemented (but only lists `status` and
  `help`, of course), as it makes use of that feature that took the main
  chunk of the Outreachy project: the function to determine unique
  prefixes of a list of strings.

- Speaking of the unique prefixes: the functionality to determine those
  is now encapsulated in the `prefix-map.c` file, and I also added a
  regression test.

- Speaking of the tests: I also implemented support for the environment
  variable `GIT_TEST_ADD_I_USE_BUILTIN`: by setting it, the test suite
  can be forced to use the built-in, or the Perl script, version of `git
  add -i`. Needless to say: by the end of this patch series, running the
  test suite with `GIT_TEST_ADD_I_USE_BUILTIN=true` will still result in
  a ton of test failures due to not-yet-implemented commands, but it
  will also demonstrate what *already* works.

- Since the main loop starts not only by showing the status, but
  refreshes the index before that, I added that, and I actually
  refactored that code into a new function
  (`repo_refresh_and_write_index()`), as it will be used a couple of
  times by the end of the complete conversion of `git add -i` into a
  built-in command.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

Documentation/config/add.txt

@@ -5,3 +5,8 @@ add.ignore-errors (deprecated)::
 	option of linkgit:git-add[1].  `add.ignore-errors` is deprecated,
 	as it does not follow the usual naming convention for configuration
 	variables.
+
+add.interactive.useBuiltin::
+	[EXPERIMENTAL] Set to `true` to use the experimental built-in
+	implementation of the interactive version of linkgit:git-add[1]
+	instead of the Perl script version. Is `false` by default.

Makefile

@@ -754,6 +754,7 @@ TEST_BUILTINS_OBJS += test-online-cpus.o
 TEST_BUILTINS_OBJS += test-parse-options.o
 TEST_BUILTINS_OBJS += test-path-utils.o
 TEST_BUILTINS_OBJS += test-pkt-line.o
+TEST_BUILTINS_OBJS += test-prefix-map.o
 TEST_BUILTINS_OBJS += test-prio-queue.o
 TEST_BUILTINS_OBJS += test-reach.o
 TEST_BUILTINS_OBJS += test-read-cache.o
@@ -848,6 +849,7 @@ LIB_H = $(shell $(FIND) . \
 	-name '*.h' -print)
 
 LIB_OBJS += abspath.o
+LIB_OBJS += add-interactive.o
 LIB_OBJS += advice.o
 LIB_OBJS += alias.o
 LIB_OBJS += alloc.o
@@ -966,6 +968,7 @@ LIB_OBJS += patch-ids.o
 LIB_OBJS += path.o
 LIB_OBJS += pathspec.o
 LIB_OBJS += pkt-line.o
+LIB_OBJS += prefix-map.o
 LIB_OBJS += preload-index.o
 LIB_OBJS += pretty.o
 LIB_OBJS += prio-queue.o