Offer a built-in version of git add -i and git add -p

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

@@ -757,6 +757,7 @@ 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
@@ -852,6 +853,8 @@ LIB_H := $(shell git ls-files '*.h' ':!t/' ':!Documentation/' 2>/dev/null || \
 	-name '*.h' -print)
 
 LIB_OBJS += abspath.o
+LIB_OBJS += add-interactive.o
+LIB_OBJS += add-patch.o
 LIB_OBJS += advice.o
 LIB_OBJS += alias.o
 LIB_OBJS += alloc.o
@@ -970,6 +973,7 @@ 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