inline lookup_replace_object() calls

In a repository without object replacement, lookup_replace_object() should
be a no-op. Check the flag "read_replace_refs" on the side of the caller,
and bypess a function call when we know we are not dealing with replacement.

Also, even when we are set up to replace objects, if we do not find any
replacement defined, flip that flag off to avoid function call overhead
for all the later object accesses.

As this change the semantics of the flag from "do we need read the
replacement definition?" to "do we need to check with the lookup table?"
the flag needs to be renamed later to something saner, e.g. "use_replace",
when the codebase is calmer, but not now.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -756,10 +756,18 @@ char *strip_path_suffix(const char *path, const char *suffix);
 int daemon_avoid_alias(const char *path);
 int offset_1st_component(const char *path);
 
+/* object replacement */
+extern void *read_sha1_file(const unsigned char *sha1, enum object_type *type, unsigned long *size);
+extern const unsigned char *do_lookup_replace_object(const unsigned char *sha1);
+static inline const unsigned char *lookup_replace_object(const unsigned char *sha1)
+{
+	if (!read_replace_refs)
+		return sha1;
+	return do_lookup_replace_object(sha1);
+}
+
 /* Read and unpack a sha1 file into memory, write memory to a sha1 file */
 extern int sha1_object_info(const unsigned char *, unsigned long *);
-extern void *read_sha1_file(const unsigned char *sha1, enum object_type *type, unsigned long *size);
-extern const unsigned char *lookup_replace_object(const unsigned char *sha1);
 extern int hash_sha1_file(const void *buf, unsigned long len, const char *type, unsigned char *sha1);
 extern int write_sha1_file(const void *buf, unsigned long len, const char *type, unsigned char *return_sha1);
 extern int pretend_sha1_file(void *, unsigned long, enum object_type, unsigned char *);

environment.c

@@ -42,7 +42,7 @@ const char *editor_program;
 const char *askpass_program;
 const char *excludes_file;
 enum auto_crlf auto_crlf = AUTO_CRLF_FALSE;
-int read_replace_refs = 1;
+int read_replace_refs = 1; /* NEEDSWORK: rename to use_replace_refs */
 enum eol eol = EOL_UNSET;
 enum safe_crlf safe_crlf = SAFE_CRLF_WARN;
 unsigned whitespace_rule_cfg = WS_DEFAULT_RULE;

replace_object.c

@@ -85,12 +85,14 @@ static void prepare_replace_object(void)
 
 	for_each_replace_ref(register_replace_ref, NULL);
 	replace_object_prepared = 1;
+	if (!replace_object_nr)
+		read_replace_refs = 0;
 }
 
 /* We allow "recursive" replacement. Only within reason, though */
 #define MAXREPLACEDEPTH 5
 
-const unsigned char *lookup_replace_object(const unsigned char *sha1)
+const unsigned char *do_lookup_replace_object(const unsigned char *sha1)
 {
 	int pos, depth = MAXREPLACEDEPTH;
 	const unsigned char *cur = sha1;