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/unshare.rs

@@ -55,6 +55,14 @@ fn copy_dir_recursive(src: &Path, dest: &Path) -> io::Result<()> {
             }
         };
 
+        // 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 tools that
+        // are not declared build-dependencies (e.g. help2man).
+        if metadata.is_dir() && super::is_vcs_dir_name(&entry.file_name()) {
+            continue;
+        }
+
         if metadata.file_type().is_symlink() {
             let target = std::fs::read_link(&src_path).map_err(|e| {
                 io::Error::new(
@@ -428,6 +436,20 @@ impl UnshareDriver {
             .lock()
             .unwrap()
             .push(dest_path.to_path_buf());
+
+        // The overlay exposes the source tree verbatim: strip VCS metadata
+        // (e.g. '.git') whose presence would flip autotools 'building from
+        // git' detection and activate maintainer-only regeneration rules
+        // (see is_vcs_dir_name). Removals land as whiteouts in the upper
+        // layer, leaving the lowerdir untouched.
+        if let Err(e) = super::prune_vcs_dirs(dest_path) {
+            log::warn!(
+                "Failed to prune VCS metadata from '{}': {}",
+                dest_path.display(),
+                e
+            );
+        }
+
         Ok(())
     }