docs & comments: replace mentions of "git-add--interactive.perl"

Now that we've removed "git-add--interactive.perl" let's replace
mentions of it with "add-interactive.c". In the case of the "git add"
documentation we were using it as an example filename, so the mention
wasn't wrong, but using a dead file is slightly confusing.

The "borrowed" comment here likewise isn't wrong, but let's mention
the successor file instead. In the case of pathspec.c the implied TODO
item should refer to the current code (and the comment may not even be
current, I didn't check).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-add.txt

@@ -274,7 +274,7 @@ status::
 ------------
               staged     unstaged path
      1:       binary      nothing foo.png
-     2:     +403/-35        +1/-1 git-add--interactive.perl
+     2:     +403/-35        +1/-1 add-interactive.c
 ------------
 +
 It shows that foo.png has differences from HEAD (but that is
@@ -282,7 +282,7 @@ binary so line count cannot be shown) and there is no
 difference between indexed copy and the working tree
 version (if the working tree version were also different,
 'binary' would have been shown in place of 'nothing').  The
-other file, git-add{litdd}interactive.perl, has 403 lines added
+other file, add-interactive.c, has 403 lines added
 and 35 lines deleted if you commit what is in the index, but
 working tree file has further modifications (one addition and
 one deletion).
@@ -303,7 +303,7 @@ like this:
 ------------
            staged     unstaged path
   1:       binary      nothing foo.png
-* 2:     +403/-35        +1/-1 git-add--interactive.perl
+* 2:     +403/-35        +1/-1 add-interactive.c
 ------------
 +
 To remove selection, prefix the input with `-`

builtin/clean.c

@@ -560,7 +560,7 @@ static int parse_choice(struct menu_stuff *menu_stuff,
 
 /*
  * Implement a git-add-interactive compatible UI, which is borrowed
- * from git-add--interactive.perl.
+ * from add-interactive.c.
  *
  * Return value:
  *

pathspec.c

@@ -545,7 +545,7 @@ static void NORETURN unsupported_magic(const char *pattern,
 	}
 	/*
 	 * We may want to substitute "this command" with a command
-	 * name. E.g. when add--interactive dies when running
+	 * name. E.g. when "git add -p" or "git add -i" dies when running
 	 * "checkout -p"
 	 */
 	die(_("%s: pathspec magic not supported by this command: %s"),