Fix escaping in generated carton-env

Escape conditional CARTON_DATA_DIR substitution in generated carton-env,
so it is substituted at the time of execution of code generated by
carton-env, not at the time of carton-env execution.
Substitute package name in carton.spec

Substitute package name in carton.spec install section instead of
hardcoding it.
Remove explicit checks for tarball/spec presence

Remove explicit "test -f" checks for tarball/spec presence in
distribution directory as they don't work with multiple files and "ln"
in loops would fail if there are none, anyway.
Use short options to git tag in test_git.sh

Use short options instead of long options to "git tag" in test_git.sh
for compatibility with older versions of git.