help.c: Add support for htmldir relative to git_exec_path()

If htmldir (in the Makefile) is a relative path, this path will now be
interpreted relative to git_exec_path.  This can be used to create an
installation that can be moved to a different directory without
re-compiling.  The Windows installer (msysgit) is an example for such
a setup.

Note that the Makefile maps htmldir to the define GIT_HTML_PATH.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

help.c

@@ -633,13 +633,15 @@ 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);
 
 	/* Check that we have a git documentation directory. */
-	if (stat(GIT_HTML_PATH "/git.html", &st) || !S_ISREG(st.st_mode))
-		die("'%s': not a documentation directory.", GIT_HTML_PATH);
+	if (stat(mkpath("%s/git.html", html_path), &st)
+	    || !S_ISREG(st.st_mode))
+		die("'%s': not a documentation directory.", html_path);
 
 	strbuf_init(page_path, 0);
-	strbuf_addf(page_path, GIT_HTML_PATH "/%s.html", page);
+	strbuf_addf(page_path, "%s/%s.html", html_path, page);
 }
 
 static void show_html_page(const char *git_cmd)