tests: Add a test for the behavior of 'hg export -- -2'

fix disappearing symlinks [issue1509]

test-merge-tools: Don't use tac "tac" was a bad tool - is not available on all platforms. The test now only uses the "tools" true, false, head, echo, cat and /bin/cat

merge: mark kept local files as readded on linear update (issue539)

Use shutil.copystat in copyfile().

Update tcsh completion scripts with new commands and for new help output. Also make the build script executable.