deb: strip VCS metadata from prepared build trees

Copying a git checkout verbatim into /tmp/pkh-build-*/ ships '.git',
flipping autotools' building-from-VCS detection (GNU hello's
BUILD_FROM_GIT): the shipped man page gets cleaned and regenerated
via help2man, which is correctly not in Build-Depends. Skip VCS
dirs (.git/.hg/.svn/.bzr/CVS) in local/unshare/ssh copies, and
prune them after an unshare overlayfs mount.

src/context/local.rs

@@ -173,6 +173,15 @@ fn copy_dir_recursive(src: &Path, dest: &Path) -> io::Result<()> {
         for entry in std::fs::read_dir(src)? {
             let entry = entry?;
             let path = entry.path();
+            // Never ship VCS metadata into the build tree: its presence
+            // flips autotools 'building from VCS' detection (see
+            // is_vcs_dir_name) and activates maintainer-only regeneration
+            // rules requiring undeclared tools (e.g. help2man).
+            if path.symlink_metadata().map(|m| m.is_dir()).unwrap_or(false)
+                && super::is_vcs_dir_name(&entry.file_name())
+            {
+                continue;
+            }
             let dest_path = dest.join(entry.file_name());
             copy_dir_recursive(&path, &dest_path)?;
         }