scripts/oe-git-archive: fix non-existent key referencing error

Without installing gitpython package, oe-git-archive will face error
below, where it was referencing key that was non-existent inside
metadata object.

Traceback (most recent call last):
  File "<poky_dir>/scripts/oe-git-archive", line 271, in <module>
    sys.exit(main())
  File "<poky_dir>/scripts/oe-git-archive", line 229, in main
    'commit_count': metadata['layers']['meta']['commit_count'],
KeyError: 'commit_count'

Fix this error by adding exception catch when referencing
non-existent key (based on inputs provided by Richard Purdie).

[YOCTO# 13082]

(From OE-Core rev: 9a3cc9b8523b78dda6c3f3f2e12798b2b907d7e5)

(From OE-Core rev: f51e59bb6d571606cf887a3f623380cc3516c5a2)

Signed-off-by: Yeoh Ee Peng <ee.peng.yeoh@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/oe-git-archive

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 #
 # Helper script for committing data to git and pushing upstream
 #
@@ -208,6 +208,13 @@ def parse_args(argv):
                         help="Data to commit")
     return parser.parse_args(argv)
 
+def get_nested(d, list_of_keys):
+    try:
+        for k in list_of_keys:
+            d = d[k]
+        return d
+    except KeyError:
+        return ""
 
 def main(argv=None):
     """Script entry point"""
@@ -223,11 +230,11 @@ def main(argv=None):
 
         # Get keywords to be used in tag and branch names and messages
         metadata = metadata_from_bb()
-        keywords = {'hostname': metadata['hostname'],
-                    'branch': metadata['layers']['meta']['branch'],
-                    'commit': metadata['layers']['meta']['commit'],
-                    'commit_count': metadata['layers']['meta']['commit_count'],
-                    'machine': metadata['config']['MACHINE']}
+        keywords = {'hostname': get_nested(metadata, ['hostname']),
+                    'branch': get_nested(metadata, ['layers', 'meta', 'branch']),
+                    'commit': get_nested(metadata, ['layers', 'meta', 'commit']),
+                    'commit_count': get_nested(metadata, ['layers', 'meta', 'commit_count']),
+                    'machine': get_nested(metadata, ['config', 'MACHINE'])}
 
         # Expand strings early in order to avoid getting into inconsistent
         # state (e.g. no tag even if data was committed)