Convert "struct cache_entry *" to "const ..." wherever possible

Convert "struct cache_entry *" to "const ..." wherever possible

I attempted to make index_state->cache[] a "const struct cache_entry **"
to find out how existing entries in index are modified and where. The
question I have is what do we do if we really need to keep track of on-disk
changes in the index. The result is

 - diff-lib.c: setting CE_UPTODATE

 - name-hash.c: setting CE_HASHED

 - preload-index.c, read-cache.c, unpack-trees.c and
   builtin/update-index: obvious

 - entry.c: write_entry() may refresh the checked out entry via
   fill_stat_cache_info(). This causes "non-const struct cache_entry
   *" in builtin/apply.c, builtin/checkout-index.c and
   builtin/checkout.c

 - builtin/ls-files.c: --with-tree changes stagemask and may set
   CE_UPDATE

Of these, write_entry() and its call sites are probably most
interesting because it modifies on-disk info. But this is stat info
and can be retrieved via refresh, at least for porcelain
commands. Other just uses ce_flags for local purposes.

So, keeping track of "dirty" entries is just a matter of setting a
flag in index modification functions exposed by read-cache.c. Except
unpack-trees, the rest of the code base does not do anything funny
behind read-cache's back.

The actual patch is less valueable than the summary above. But if
anyone wants to re-identify the above sites. Applying this patch, then
this:

    diff --git a/cache.h b/cache.h
    index 430d021..1692891 100644
    --- a/cache.h
    +++ b/cache.h
    @@ -267,7 +267,7 @@ static inline unsigned int canon_mode(unsigned int mode)
     #define cache_entry_size(len) (offsetof(struct cache_entry,name) + (len) + 1)

     struct index_state {
    -	struct cache_entry **cache;
    +	const struct cache_entry **cache;
     	unsigned int version;
     	unsigned int cache_nr, cache_alloc, cache_changed;
     	struct string_list *resolve_undo;

will help quickly identify them without bogus warnings.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>