Steal a few more lines from git's pager code and only pass the hardcoded
"-FRSX" flags if the user has not set the LESS environmen variable to some
other variable, thus behaving more closely to what the user would expect.

This is useful for avoiding crashing on "cdiff -l" (on Python 3) in
https://github.com/myint/cppclean.

Also handle unknown encodings gracefully rather than crashing.

Have to create pager Popen object before the translator Popen object
in PatchStreamForwarder, otherwise the `stdin=subprocess.PIPE` would cause
trouble to the translator pipe (select() never see EOF after input stream
ended), most likely python bug 12607 (http://bugs.python.org/issue12607)
which was fixed in python 2.7.3.

blocking mode with select() and bufsize=1, buggy in python 2.x:
close() does not notify select() for EOF

Also handle abruptly closed pipe gracefully.

Use OptionGroup for hack

- Close issue #26 (user can now use cdiff -s -U5 --cached)

Use `--no-ext-diff` to disable GIT_EXTERNAL_DIFF and diff.external which might
break cdiff output

When the git diff command is customized through setting GIT_EXTERNAL_DIFF in the environment, the output of cdiff is broken and defaults to the output of the tool git has been configured to use.

Temporarily unsetting this variable when cdiff invokes git diff fixes the issue.

If need to support normal diff, writing a ndiff to udiff translator
makes more sense