dir.c: free "ident" and "exclude_per_dir" in "struct untracked_cache"

dir.c: free "ident" and "exclude_per_dir" in "struct untracked_cache"

When the "ident" member of the structure was added in
1e8fef609e7 (untracked cache: guard and disable on system changes,
2015-03-08) this function wasn't updated to free it. Let's do so.

Let's also free the "exclude_per_dir" memory we've been leaking
since[1], while making sure not to free() the constant ".gitignore"
string we add by default[2].

As we now have three struct members we're freeing let's change
free_untracked_cache() to return early if "uc" isn't defined. We won't
hand it to free() now, but that was just for convenience, once we're
dealing with >=2 struct members this pattern is more convenient.

1. f9e6c649589 (untracked cache: load from UNTR index extension,
   2015-03-08)
2. 039bc64e886 (core.excludesfile clean-up, 2007-11-14)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>