help: check early if we have a command, if not try a documentation topic

Before this patch, something like "git help tutorial" did not work,
people had to use "git help gittutorial" which is not very intuitive.

This patch uses the "is_git_command" function to test early if the
argument passed to "git help" is a git command, and if this is not the
case then we prefix the argument with "git" instead of "git-".

This way, things like "git help tutorial" or "git help glossary" will
work fine.

The little downside of this patch is that the "is_git_command" is a
little bit slow.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

help.c

@@ -527,20 +527,26 @@ 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)
 		return "git";
 	else if (!prefixcmp(git_cmd, "git"))
 		return git_cmd;
-	else {
-		int page_len = strlen(git_cmd) + 4;
-		char *p = xmalloc(page_len + 1);
-		strcpy(p, "git-");
-		strcpy(p + 4, git_cmd);
-		p[page_len] = 0;
-		return p;
-	}
+	else if (is_git_command(git_cmd))
+		return prepend("git-", git_cmd);
+	else
+		return prepend("git", git_cmd);
 }
 
 static void setup_man_path(void)