[cygwin][git] Use git difftool to keep git diff behavior.

Since there is git difftool to launch vimdiff for visual diff, no
necessary to keey below configurations:

1. function qgitdiff()
2. bin/git_diff_wrapper
3. .gitconfig:[diff], [pager]

Just use:
$ git difftool
Then there will be a better interface than git_diff_wrapper.
[cygwin][vim] Re-design session-auto feature. Using vi-proj to trigger.

New "vi-proj" alias, which enables session-auto feature.
Modified "revi" function, which uses vi-proj to create new project with
previously opened files.

Use cases:
\#1. Create new project
     $ vi-proj
     -> then open files in vim
     -> quit, then session will be auto-saved
     $ vi-proj
     -> the last session will be auto-loaded
\#2. Create new project with opening files
     $ vi-proj file1 file2
\#3. Open last auto-saved session
     $ vi-proj
\#4. Reload .vimrc and auto-open files in saved session.
     $ revi
[cygwin][vim] New bash command lviproj to view all files in current session.

Also, try to fix session auto-save error message in .vimrc.