filter-repo: partial repo filtering considerations

Fix a few issues and add a token testcase for partial repo filtering.
Add a note about how I think this is not a particularly interesting or
core usecase for filter-repo, even if I have put some good effort into
the fast-export side to ensure it worked.  If there is a core usecase
that can be addressed without causing usability problems (particularly
the "don't mix old and new history" edict for normal rewrites), then
I'll be happy to add more testcases, document it better, etc.

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

README.md

@@ -953,6 +953,18 @@ the user when it detects an issue:
     such as `-M` or `-C` would break assumptions used in other places of
     filter-repo.
 
+  * Partial-repo filtering does not mesh well with filter-repo's "avoid
+    mixing old and new history" design.  filter-repo has some capability in
+    this area but it is undocumented, mostly untested, and may require
+    multiple non-obvious flags to be set to make sane use of it.  While
+    there might be valid usecases for partial-repo filtering, the only ones
+    I've run into in the wild are sidestepping filter-branch's insanely
+    slow execution on commits that would not be changed by the filters in
+    question anyway (which is largely irrelevant since filter-repo is
+    multiple orders of magnitude faster), or to do operations better suited
+    to git-rebase(1) and which rebase grew special options for years ago
+    (e.g. the `--signoff` option).
+
 ### Comments on reversibility
 
 Some people are interested in reversibility of of a rewrite; e.g. rewrite

git-filter-repo

@@ -1521,8 +1521,8 @@ class FilteringOptions(object):
                "Will not modify your repo."))
 
     refs = parser.add_argument_group(title=_("Git References"))
-    refs.add_argument('--refs', action='store_const', const=['--all'],
-                      default=['--all'], help=argparse.SUPPRESS)
+    refs.add_argument('--refs', nargs='*', default=['--all'],
+                      help=argparse.SUPPRESS)
 
     path = parser.add_argument_group(title=_("Filtering based on paths "
                                              "(see also --filename-callback)"))
@@ -3159,7 +3159,7 @@ class RepoFilter(object):
       location = ['-C', self._args.source] if self._args.source else []
       fep_cmd = ['git'] + location + ['fast-export', '--show-original-ids',
                  '--signed-tags=strip', '--tag-of-filtered-object=rewrite',
-                 '--fake-missing-tagger'
+                 '--fake-missing-tagger', '--reference-excluded-parents'
                  ] + extra_flags + self._args.refs
       self._fep = subprocess.Popen(fep_cmd, bufsize=-1, stdout=subprocess.PIPE)
       self._input = self._fep.stdout

t/t9390-filter-repo.sh

@@ -1025,6 +1025,23 @@ test_expect_success '--target' '
 	test 2 = $(git -C target rev-list --count master)
 '
 
+test_expect_success '--refs' '
+	git init refs &&
+	(
+		cd refs &&
+		git checkout -b other &&
+		echo hello >world &&
+		git add world &&
+		git commit -m init
+	) &&
+	git -C refs rev-parse other >refs/expect &&
+	git -C analyze_me rev-parse master >refs/expect &&
+	git filter-repo --source analyze_me --target refs --refs master --force &&
+	git -C refs rev-parse other >refs/actual &&
+	git -C refs rev-parse master >refs/actual &&
+	test_cmp refs/expect refs/actual
+'
+
 test_expect_success 'reset to specific refs' '
 	test_create_repo reset_to_specific_refs &&
 	(