While trying to debug an issue, I discovered that the tests for illegal
arguments depended on the prior suite having run and started a session.
Let's remove that unintentional dependency by starting our own session
before each test.
New ex commands: 'tcd', 'tchdir'
Changed Vimscript functions: 'haslocaldir', 'getcwd'
The ex-commands ':tcd' and ':tchdir' are the tab-local equivalents of
':lcd' and ':lchdir'. There are no new Vimscript functions introduced,
instead the functions 'haslocaldir' and 'getcwd' take in optional
arguments. See the documentation for details
Since there is now different levels of local directory a simple boolean
at source level is no longer sufficient; a new enumeration type is used
for the scope-level from now on.
The documentation has been accommodated for these new commands and
functional tests have been written to test the feature.
