Change revert-buffer-function to buffer local variable.

Change revert-buffer-function from global variable to to buffer local
variable in yagist-list-mode. IF revert-buffer-function is global
variable, `revert-buffer` function is called in any other buffer each
time yagist-list-revert-buffer is called too in any other buffer.

Step to reproduce:
1. M-x yagist-list
2. switch to any other buffer (e.g *scratch*)
3. M-x revert-buffer