builtin-help: fallback to GIT_MAN_VIEWER before man

In some situations it is useful to be able to switch viewers via the
environment, e.g. in Emacs shell buffers.  So check the GIT_MAN_VIEWER
environment variable and try it before falling back to "man".

Signed-off-by: Romain Francoise <romain@orebokech.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-help.txt

@@ -112,7 +112,9 @@ For example, this configuration:
 will try to use konqueror first. But this may fail (for example if
 DISPLAY is not set) and in that case emacs' woman mode will be tried.
 
-If everything fails the 'man' program will be tried anyway.
+If everything fails, or if no viewer is configured, the viewer specified
+in the GIT_MAN_VIEWER environment variable will be tried.  If that
+fails too, the 'man' program will be tried anyway.
 
 man.<tool>.path
 ~~~~~~~~~~~~~~~

builtin-help.c

@@ -361,12 +361,15 @@ static void show_man_page(const char *git_cmd)
 {
 	struct man_viewer_list *viewer;
 	const char *page = cmd_to_page(git_cmd);
+	const char *fallback = getenv("GIT_MAN_VIEWER");
 
 	setup_man_path();
 	for (viewer = man_viewer_list; viewer; viewer = viewer->next)
 	{
 		exec_viewer(viewer->name, page); /* will return when unable */
 	}
+	if (fallback)
+		exec_viewer(fallback, page);
 	exec_viewer("man", page);
 	die("no man viewer handled the request");
 }