[PATCH v2] fast-import: disallow "." and ".." path components

From: Elijah Newren <newren@gmail.com>

If a user specified e.g.
   M 100644 :1 ../some-file
then fast-import previously would happily create a git history where
there is a tree in the top-level directory named "..", and with a file
inside that directory named "some-file".  The top-level ".." directory
causes problems.  While git checkout will die with errors and fsck will
report hasDotdot problems, the user is going to have problems trying to
remove the problematic file.  Simply avoid creating this bad history in
the first place.

Signed-off-by: Elijah Newren <newren@gmail.com>
---
    fast-import: disallow "." and ".." path components
    
    Changes since v1:
    
     * make use of is_dot_or_dotdot() from dir.h
     * fix style issue

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1831%2Fnewren%2Fdisallow-dotdot-fast-import-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1831/newren/disallow-dotdot-fast-import-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1831

Range-diff vs v1:

 1:  86ea3df0351 ! 1:  447b6794a4a fast-import: disallow "." and ".." path components
     @@ builtin/fast-import.c: static int tree_content_set(
       		root->tree = t = grow_tree_content(t, t->entry_count);
       	e = new_tree_entry();
       	e->name = to_atom(p, n);
     -+	if (!strcmp(e->name->str_dat, ".") || !strcmp(e->name->str_dat, "..")) {
     ++	if (is_dot_or_dotdot(e->name->str_dat))
      +		die("path %s contains invalid component", p);
     -+	}
       	e->versions[0].mode = 0;
       	oidclr(&e->versions[0].oid, the_repository->hash_algo);
       	t->entries[t->entry_count++] = e;

 builtin/fast-import.c  |  2 ++
 t/t9300-fast-import.sh | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+)