git.py: Add configurable warning for inconsistent-submodule

This follows the implementation of configurable warnings.

buildstream/plugins/sources/git.py

@@ -68,6 +68,12 @@ git - stage files from a git repository
        url: upstream:baz.git
        checkout: False
 
+**Configurable Warnings:**
+
+This plugin provides the following configurable warnings:
+
+- 'git:inconsistent-submodule' - A submodule was found to be missing from the underlying git repository.
+
 """
 
 import os
@@ -84,6 +90,9 @@ from buildstream import utils
 
 GIT_MODULES = '.gitmodules'
 
+# Warnings
+INCONSISTENT_SUBMODULE = "inconsistent-submodules"
+
 
 # Because of handling of submodules, we maintain a GitMirror
 # for the primary git source and also for each submodule it
@@ -283,7 +292,7 @@ class GitMirror(SourceFetcher):
                      "underlying git repository with `git submodule add`."
 
             self.source.warn("{}: Ignoring inconsistent submodule '{}'"
-                             .format(self.source, submodule), detail=detail)
+                             .format(self.source, submodule), detail=detail, warning_token=INCONSISTENT_SUBMODULE)
 
             return None
 
@@ -350,6 +359,9 @@ class GitSource(Source):
             return Consistency.RESOLVED
         return Consistency.INCONSISTENT
 
+    def get_warnings(self):
+        return [INCONSISTENT_SUBMODULE]
+
     def load_ref(self, node):
         self.mirror.ref = self.node_get_member(node, str, 'ref', None)