externalsrc: Fix parsing error with devtool non-git sources

If srcdir is under poky directory (e.g. devtool poky/build/workspace/sources)
and is not a git repository then ${@srctree_hash_files(d)} will run "git
rev-parse --git-dir" and detect poky directory as git-dir and run "'git', 'add',
'-A', '.'], cwd=s_dir" trying to add srcdir but build dir is in .gitignore and
latest git will fail with "The following paths are ignored by one of your
.gitignore files: build" which will end with "ExpansionError during parsing".

In this commit I added a check if git_dir is the same as git-dir from
TOPDIR (which will detect poky directory) and if yes, then treat srcdir
as non-git sources.

(From OE-Core rev: 95fbac8dcad6c93f4c9737e9fe13e92ab6befa09)

Signed-off-by: Tomasz Dziendzielski <tomasz.dziendzielski@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/externalsrc.bbclass

@@ -197,6 +197,10 @@ def srctree_hash_files(d, srcdir=None):
     try:
         git_dir = os.path.join(s_dir,
             subprocess.check_output(['git', '-C', s_dir, 'rev-parse', '--git-dir'], stderr=subprocess.DEVNULL).decode("utf-8").rstrip())
+        top_git_dir = os.path.join(s_dir, subprocess.check_output(['git', '-C', d.getVar("TOPDIR"), 'rev-parse', '--git-dir'],
+            stderr=subprocess.DEVNULL).decode("utf-8").rstrip())
+        if git_dir == top_git_dir:
+            git_dir = None
     except subprocess.CalledProcessError:
         pass