git-pull.sh: prefer invoking "git <command>" over "git-<command>"

14e5d40c (pull: Fix parsing of -X<option>, 2010-01-17) added the lines
containing "git-push" and "git-merge", even though the prevelant style
at the time was to use the unhyphenated "git <command>" form.  Fix
this.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
git-pull.sh: escape quotes within quotes

Although it works fine, escape quotes within quotes to avoid confusing
humans and syntax-highlighters.  The shell-mode distributed with Emacs
syntax highlights in unexpected ways without this patch.
git-pull.sh: introduce --[no-]autostash and pull.autostash