strbuf: add strbuf_tolower function

This is a convenience wrapper to call tolower on each
character of the string.

This makes config's lowercase() function obsolete, though
note that because we have a strbuf, we are careful to
operate over the whole strbuf, rather than assuming that a
NUL is the end-of-string.

We could continue to offer a pure-string lowercase, but
there would be no callers (in most pure-string cases, we
actually duplicate and lowercase the duplicate, for which we
have the xstrdup_tolower wrapper).
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/technical/api-strbuf.txt

@@ -125,6 +125,10 @@ Functions
 
 	Strip whitespace from the end of a string.
 
+`strbuf_tolower`::
+
+	Lowercase each character in the buffer using `tolower`.
+
 `strbuf_cmp`::
 
 	Compare two buffers. Returns an integer less than, equal to, or greater

config.c

@@ -147,12 +147,6 @@ int git_config_include(const char *var, const char *value, void *data)
 	return ret;
 }
 
-static void lowercase(char *p)
-{
-	for (; *p; p++)
-		*p = tolower(*p);
-}
-
 void git_config_push_parameter(const char *text)
 {
 	struct strbuf env = STRBUF_INIT;
@@ -180,7 +174,7 @@ int git_config_parse_parameter(const char *text,
 		strbuf_list_free(pair);
 		return error("bogus config parameter: %s", text);
 	}
-	lowercase(pair[0]->buf);
+	strbuf_tolower(pair[0]);
 	if (fn(pair[0]->buf, pair[1] ? pair[1]->buf : NULL, data) < 0) {
 		strbuf_list_free(pair);
 		return -1;

strbuf.c

@@ -106,6 +106,13 @@ void strbuf_ltrim(struct strbuf *sb)
 	sb->buf[sb->len] = '\0';
 }
 
+void strbuf_tolower(struct strbuf *sb)
+{
+	char *p = sb->buf, *end = sb->buf + sb->len;
+	for (; p < end; p++)
+		*p = tolower(*p);
+}
+
 struct strbuf **strbuf_split_buf(const char *str, size_t slen,
 				 int terminator, int max)
 {

strbuf.h

@@ -45,6 +45,7 @@ static inline void strbuf_setlen(struct strbuf *sb, size_t len)
 extern void strbuf_trim(struct strbuf *);
 extern void strbuf_rtrim(struct strbuf *);
 extern void strbuf_ltrim(struct strbuf *);
+extern void strbuf_tolower(struct strbuf *sb);
 extern int strbuf_cmp(const struct strbuf *, const struct strbuf *);
 
 /*