help: drop prepend function in favor of xstrfmt

This function predates xstrfmt, and its functionality is a
subset. Let's just use xstrfmt.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/help.c

@@ -295,16 +295,6 @@ static int is_git_command(const char *s)
 		is_in_cmdlist(&other_cmds, s);
 }
 
-static const char *prepend(const char *prefix, const char *cmd)
-{
-	size_t pre_len = strlen(prefix);
-	size_t cmd_len = strlen(cmd);
-	char *p = xmalloc(pre_len + cmd_len + 1);
-	memcpy(p, prefix, pre_len);
-	strcpy(p + pre_len, cmd);
-	return p;
-}
-
 static const char *cmd_to_page(const char *git_cmd)
 {
 	if (!git_cmd)
@@ -312,9 +302,9 @@ static const char *cmd_to_page(const char *git_cmd)
 	else if (starts_with(git_cmd, "git"))
 		return git_cmd;
 	else if (is_git_command(git_cmd))
-		return prepend("git-", git_cmd);
+		return xstrfmt("git-%s", git_cmd);
 	else
-		return prepend("git", git_cmd);
+		return xstrfmt("git%s", git_cmd);
 }
 
 static void setup_man_path(void)