filter-repo: fix incremental imports and add a test

The AncestryGraph setup assumed we had previously seen all commits which
would be used as parents; that interacted badly with doing an
incremental import.  Add a function which can be used to record external
commits, each of which we'll treat like a root commit (i.e. depth 1 and
having no parents of its own).  Add a test to prevent regressions.

Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -151,6 +151,17 @@ class AncestryGraph(object):
     # than the max depth of any of its ancestors.
     self.graph = {}
 
+  def record_external_commits(self, external_commits):
+    """
+    Record in graph that each commit in external_commits exists, and is
+    treated as a root commit with no parents.
+    """
+    for c in external_commits:
+      if c not in self.value:
+        self.cur_value += 1
+        self.value[c] = self.cur_value
+        self.graph[self.cur_value] = (1, [])
+
   def add_commit_and_parents(self, commit, parents):
     """
     Record in graph that commit has the given parents.  parents _MUST_ have
@@ -1441,6 +1452,10 @@ class FastExportFilter(object):
       _IDS.record_rename(id_, commit.id)
 
     # Record ancestry graph
+    external_parents = [p for p in commit.get_parents()
+                        if not isinstance(p, int)]
+    self._graph.record_external_commits(external_parents)
+    self._orig_graph.record_external_commits(external_parents)
     self._graph.add_commit_and_parents(commit.id, commit.get_parents())
     self._orig_graph.add_commit_and_parents(id_, orig_parents)
 

t/t9390-filter-repo.sh

@@ -788,4 +788,16 @@ test_expect_success 'mailmap sanity checks' '
 	)
 '
 
+test_expect_success 'incremental import' '
+	(
+		git clone file://"$(pwd)"/analyze_me incremental &&
+		cd incremental &&
+
+		original=$(git rev-parse master) &&
+		git fast-export --reference-excluded-parents master~2..master \
+			| git filter-repo --stdin --refname-callback "return \"develop\"" &&
+		test "$(git rev-parse develop)" = "$original"
+	)
+'
+
 test_done