lib/buildcfg: Share common clean/dirty layer function

The comments even say this was copy/paste code. Move to a
shared library function.

(From OE-Core rev: ac3de2f850a418673b87e1c454970cb099e191b0)

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

meta/lib/oe/buildcfg.py

@@ -37,4 +37,17 @@ def get_metadata_git_revision(path, d):
     except bb.process.ExecutionError:
         rev = '<unknown>'
     return rev.strip()
-         
+
+def is_layer_modified(path):
+    try:
+        subprocess.check_output("""cd %s; export PSEUDO_UNLOAD=1; set -e;
+                                git diff --quiet --no-ext-diff
+                                git diff --quiet --no-ext-diff --cached""" % path,
+                                shell=True,
+                                stderr=subprocess.STDOUT)
+        return ""
+    except subprocess.CalledProcessError as ex:
+        # Silently treat errors as "modified", without checking for the
+        # (expected) return code 1 in a modified git repo. For example, we get
+        # output and a 129 return code when a layer isn't a git repo at all.
+        return " -- modified"