filter-repo: suggest --no-local when cloning local repos

Cloning local repos by default makes a bunch of hardlinks, giving you a
non-packed repository, and leading folks to use and suggest --force.
That, of course, bypasses the important fresh clone checks to prevent
people from accidentally and irrecoverably deleting their non-backed-up
data.  Let's make it easier for people to avoid (and suggest) that
mistake.

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

Documentation/git-filter-repo.txt

@@ -277,7 +277,9 @@ Miscellaneous options
 --force::
 -f::
 	Rewrite history even if the current repo does not look like a fresh
-	clone.
+	clone.  Note that when cloning repos on a local filesystem, it is
+	better to pass `--no-local` to git clone than passing `--force` to
+	git-filter-repo.
 
 --partial::
 	Do a partial history rewrite, resulting in the mixture of old and

git-filter-repo

@@ -2846,11 +2846,20 @@ class RepoFilter(object):
   @staticmethod
   def sanity_check(refs, is_bare):
     def abort(reason):
+      try:
+        cmd = 'git config remote.origin.url'
+        output = subproc.check_output(cmd.split()).strip()
+      except subprocess.CalledProcessError as e:
+        output = None
+      msg = ""
+      if os.path.isdir(output):
+        msg = _("Note: when cloning local repositories, you need to pass\n"
+                "      --no-local to git clone to avoid this issue.\n")
       raise SystemExit(
         _("Aborting: Refusing to overwrite repo history since this does not\n"
           "look like a fresh clone.\n"
-          "  (%s)\n"
-          "To override, use --force.") % reason)
+          "  (%s)\n%s"
+          "To override, use --force.") % (reason, msg))
 
     # Make sure repo is fully packed, just like a fresh clone would be.
     # Note that transfer.unpackLimit defaults to 100, meaning that a

t/t9390-filter-repo.sh

@@ -1044,6 +1044,18 @@ test_expect_success 'startup sanity checks' '
 		git update-ref -m funsies refs/remotes/origin/master refs/heads/master~1 &&
 		test_must_fail git filter-repo --path numbers 2>../err &&
 		test_i18ngrep "refs/heads/master does not match refs/remotes/origin/master" ../err &&
+		rm ../err &&
+
+		cd ../ &&
+		git -C analyze_me gc &&
+		echo foobar | git -C analyze_me hash-object -w --stdin &&
+		git clone analyze_me startup_sanity_checks2 &&
+		cd startup_sanity_checks2 &&
+
+		echo foobar | git hash-object -w --stdin &&
+		test_must_fail git filter-repo --path numbers 2>../err &&
+		test_i18ngrep "expected freshly packed repo" ../err &&
+		test_i18ngrep "when cloning local repositories" ../err &&
 		rm ../err
 	)
 '