[cygwin][git][vim] Mainly, use vimdiff as default git diff.

1. In .gitconfig, use vimdiff to show every modified files' changes.

2. Auto save-load session (vim) will have problems since vimdiff will
run multiple times in a git diff. Disable auto save-load session and use
key mapping for session save and load.

For vim:
<F2> save session
<F3> load session