Add config variable to set HTML path for git-help --web

If set in git-config, help.htmlpath overrides system_path(GIT_HTML_PATH)
which was compiled in. This allows users to repoint system-wide git at
their own copy of the documentation without recompiling.

Signed-off-by: Chris Webb <chris@arachsys.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/help.c

@@ -30,6 +30,8 @@ enum help_format {
 	HELP_FORMAT_WEB
 };
 
+static const char *html_path;
+
 static int show_all = 0;
 static unsigned int colopts;
 static enum help_format help_format = HELP_FORMAT_NONE;
@@ -261,6 +263,12 @@ static int git_help_config(const char *var, const char *value, void *cb)
 		help_format = parse_help_format(value);
 		return 0;
 	}
+	if (!strcmp(var, "help.htmlpath")) {
+		if (!value)
+			return config_error_nonbool(var);
+		html_path = xstrdup(value);
+		return 0;
+	}
 	if (!strcmp(var, "man.viewer")) {
 		if (!value)
 			return config_error_nonbool(var);
@@ -383,7 +391,8 @@ static void show_info_page(const char *git_cmd)
 static void get_html_page_path(struct strbuf *page_path, const char *page)
 {
 	struct stat st;
-	const char *html_path = system_path(GIT_HTML_PATH);
+	if (!html_path)
+		html_path = system_path(GIT_HTML_PATH);
 
 	/* Check that we have a git documentation directory. */
 	if (stat(mkpath("%s/git.html", html_path), &st)