tg.sh: add a pair of missing quotes around @cmddir@
paging: improve paging code

Firstly, do not use mkfifo as it's not available everywhere.

Secondly, properly support the notion GIT_PAGER= and PAGER= to
indicate that no paging should occur.

Thirdly, add support for the LV environment variable.

Fourthly, only set LESS and/or LV environment variables if they
are actually unset, but not if they are set to "" (see the
setup_pager function in Git's pager.c source file).
tg-import.sh: fix typo
README: add a README.rst symbolic link to README

A hosting site that shall not be named uses this "feature"
to format the README file.
tg.sh: add more quotes in Git version checking code

It was probably okay the way it was, but just in case, to be
safe, the extra quotes do not hurt anything.
tg-export --collapse | --linearize to existing branch proceeds with --force

If the branch exists and --force is provided, pass -B to git-checkout,
available in git since v1.7.3 (2010 Sept).