build-pkg: use "git add" with --all

From the warning produced by "git add ." after removing a file with "rm":

> warning: You ran 'git add' with neither '-A (--all)' or '--ignore-removal',
> whose behaviour will change in Git 2.0 with respect to paths you removed.
> Paths like 'foo.txt' that are
> removed from your working tree are ignored with this version of Git.
>
> * 'git add --ignore-removal <pathspec>', which is the current default,
>   ignores paths you removed from your working tree.
>
> * 'git add --all <pathspec>' will let you also record the removals.
>
> Run 'git status' to check the paths you removed from your working tree.

$ git status --porcelain
 D foo.txt

$ git add --all .

$ git status --porcelain
D  foo.txt

$ git --version
git version 1.9.1

1 files changed, 2 insertions, 1 deletions

diff --git a/tools/build-pkg.lua b/tools/build-pkg.lua
index 81b0391..b3fa217 100755
--- a/tools/build-pkg.lua
+++ b/tools/build-pkg.lua
@@ -430,7 +430,7 @@ local function gitCheckout(new_branch, deps, item2index, pass_of_deps)
 end
 
 local function gitAdd()
-    os.execute(GIT .. 'add .')
+    os.execute(GIT .. 'add --all .')
 end
 
 -- return two lists of filepaths under ./usr/
@@ -442,6 +442,7 @@ local function gitStatus()
     local git_st = io.popen(GIT .. 'status --porcelain', 'r')
     for line in git_st:lines() do
         local status, file = line:match('(..) (.*)')
+        assert(status:sub(2, 2) == ' ')
         status = trim(status)
         if file:sub(1, 1) == '"' then
             -- filename with a space is quoted by git