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>

entry.c

@@ -79,7 +79,7 @@ static int create_file(const char *path, unsigned int mode)
 	return open(path, O_WRONLY | O_CREAT | O_EXCL, mode);
 }
 
-static void *read_blob_entry(struct cache_entry *ce, unsigned long *size)
+static void *read_blob_entry(const struct cache_entry *ce, unsigned long *size)
 {
 	enum object_type type;
 	void *new = read_sha1_file(ce->sha1, &type, size);
@@ -92,7 +92,7 @@ static void *read_blob_entry(struct cache_entry *ce, unsigned long *size)
 	return NULL;
 }
 
-static int open_output_fd(char *path, struct cache_entry *ce, int to_tempfile)
+static int open_output_fd(char *path, const struct cache_entry *ce, int to_tempfile)
 {
 	int symlink = (ce->ce_mode & S_IFMT) != S_IFREG;
 	if (to_tempfile) {
@@ -115,7 +115,7 @@ static int fstat_output(int fd, const struct checkout *state, struct stat *st)
 	return 0;
 }
 
-static int streaming_write_entry(struct cache_entry *ce, char *path,
+static int streaming_write_entry(const struct cache_entry *ce, char *path,
 				 struct stream_filter *filter,
 				 const struct checkout *state, int to_tempfile,
 				 int *fstat_done, struct stat *statbuf)
@@ -136,7 +136,8 @@ static int streaming_write_entry(struct cache_entry *ce, char *path,
 	return result;
 }
 
-static int write_entry(struct cache_entry *ce, char *path, const struct checkout *state, int to_tempfile)
+static int write_entry(struct cache_entry *ce,
+		       char *path, const struct checkout *state, int to_tempfile)
 {
 	unsigned int ce_mode_s_ifmt = ce->ce_mode & S_IFMT;
 	int fd, ret, fstat_done = 0;
@@ -233,7 +234,8 @@ static int check_path(const char *path, int len, struct stat *st, int skiplen)
 	return lstat(path, st);
 }
 
-int checkout_entry(struct cache_entry *ce, const struct checkout *state, char *topath)
+int checkout_entry(struct cache_entry *ce,
+		   const struct checkout *state, char *topath)
 {
 	static char path[PATH_MAX + 1];
 	struct stat st;