filter-repo: handle reset to specific ref and deletion

The reset directive can specify a commit hash for the 'from' directive,
which can be used to reset to a specify commit, or, if the hash is all
zeros, then it can be used to delete the ref.  Support such operations.

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

git-filter-repo

@@ -534,7 +534,10 @@ class Reset(_GitElement):
 
     file_.write(b'reset %s\n' % self.ref)
     if self.from_ref:
-      file_.write(b'from :%d\n' % self.from_ref)
+      if isinstance(self.from_ref, int):
+        file_.write(b'from :%d\n' % self.from_ref)
+      else:
+        file_.write(b'from %s\n' % self.from_ref)
       file_.write(b'\n')
 
 class FileChange(_GitElement):
@@ -3066,6 +3069,8 @@ class RepoFilter(object):
     # Record we've seen this ref and don't need to force a manual update
     # for it.
     self._seen_refs[reset.ref] = None
+    if isinstance(reset.from_ref, bytes) and re.match(b'^0+$', reset.from_ref):
+      del self._seen_refs[reset.ref]
 
   def results_tmp_dir(self, create_if_missing=True):
     working_dir = self._args.target or self._args.source or b'.'

t/t9390-filter-repo.sh

@@ -950,6 +950,29 @@ test_expect_success 'incremental import' '
 	)
 '
 
+test_expect_success 'reset to specific refs' '
+	test_create_repo reset_to_specific_refs &&
+	(
+		cd reset_to_specific_refs &&
+
+		git commit --allow-empty -m initial &&
+		INITIAL=$(git rev-parse HEAD) &&
+		echo "$INITIAL refs/heads/develop" >expect &&
+
+		cat >input <<-INPUT_END &&
+		reset refs/heads/develop
+		from $INITIAL
+
+		reset refs/heads/master
+		from 0000000000000000000000000000000000000000
+		INPUT_END
+
+		cat input | git filter-repo --force --stdin &&
+		git show-ref >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_expect_success 'setup handle funny characters' '
 	test_create_repo funny_chars &&
 	(