externalsrc: Use git add -A for compatibility with all git versions

I've been debugging a selftest failure on Centos7. The problem turns out
to be the elderly git version (1.8.3.1) on those systems. It means that
the system doesn't correctly checksum changed files in the source tree,
which in turn means do_compile fails to run and this leads to the following
selftest failure:

======================================================================
FAIL [141.373s]: test_devtool_buildclean (oeqa.selftest.devtool.DevtoolTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/pokybuild/yocto-autobuilder/yocto-worker/nightly-oe-selftest/build/meta/lib/oeqa/selftest/devtool.py", line 530, in test_devtool_buildclean
    assertFile(tempdir_mdadm, 'mdadm')
  File "/home/pokybuild/yocto-autobuilder/yocto-worker/nightly-oe-selftest/build/meta/lib/oeqa/selftest/devtool.py", line 497, in assertFile
    self.assertTrue(os.path.exists(f), "%r does not exist" % f)
AssertionError: False is not true : '/tmp/devtoolqag88s39z8/mdadm' does not exist

The solution is to use -A on the git add commandline which matches the behaviour
in git 2.0+ versions and resolves the problem.

(From OE-Core rev: 964e8b8cae4b28e21ade12b5effb494e459b1f0f)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/externalsrc.bbclass

@@ -190,7 +190,7 @@ def srctree_hash_files(d, srcdir=None):
             # Update our custom index
             env = os.environ.copy()
             env['GIT_INDEX_FILE'] = tmp_index.name
-            subprocess.check_output(['git', 'add', '.'], cwd=s_dir, env=env)
+            subprocess.check_output(['git', 'add', '-A', '.'], cwd=s_dir, env=env)
             sha1 = subprocess.check_output(['git', 'write-tree'], cwd=s_dir, env=env).decode("utf-8")
         with open(oe_hash_file, 'w') as fobj:
             fobj.write(sha1)