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

Without this, some of the logs will not be shown.

- Fixed a bug in diff format probe
- Handle keyboard interrupt and large diffs in non-color mode
- Code clean up

Previously, a keyboard interrupt would result in a corrupted terminal.
This would manifest when doing "cdiff --log" on somewhat large
repositories.

The user explicitly asked for a log and thus knows what he is doing.

This will use the patch log from git, svn, or hg.