Fix mksession to set localdir correctly when there are multiple tabs

[ychin]

This patch fixes an issue in mksession where local directories don't get properly saved out if there are multiple tabs. A simple repro is as follows:

edit some_file.txt
tabedit some_file2.txt
tabfirst
lcd ~/.vim
mksession ~/session.vim
qa

Then in another Vim instance:

source ~/session.vim
tablast
assert_true(!haslocaldir(), 'should not have localdir')

The assert will fire. The issue is that mksession makes new tabs using tabedit after populating the current tab's windows first. The issue is tabedit will propagate local options to the new tab, including the localdir. Instead, the fix is to pre-populate all the tabs first (similar to how the script pre-populates the window splits in ses_win_rec()).

In testing this I also found a couple bugs that I fixed:

1. The session Vim script doesn't clear all the other tabs on load. This is inconsistent to how it uses silent only to clear all the other windows first. To fix this, add silent tabonly to make sure everything is cleared on load. This also makes sure the last command in the script to re-select the active tab via tabnext will work properly.

• (I do realize this has a small chance of breaking people who got used to the old inconsistent behavior. I don't know how big that population is)

2. If the first window of first tab is a blank window, then mksession will generate a script that deletes that window, since s:wipebuf is actually the buffer for the blank window when restoring. Fix this by making sure s:wipebuf is actually not used anywhere before calling bwipe on it. See Empty first tab lost when using sessions #1282 for an existing issue calling this out.

[ychin]

I just saw from that the unit tests are failing on Windows from my change (https://ci.appveyor.com/project/chrisbra/vim/build/5845/job/5iyg6ol949fwddte). It's due to a minor difference in how lcd works on Unix systems and Windows. I'll submit a new PR to make sure it passes on Windows.

[ychin]

Oh wait you already fixed that in 81e2ac7, nevermind!

[brammool]

I just saw from that the unit tests are failing on Windows from my change (https://ci.appveyor.com/project/chrisbra/vim/build/5845/job/5iyg6ol949fwddte). It's due to a minor difference in how `lcd` works on Unix systems and Windows. I'll submit a new PR to make sure it passes on Windows.

Yeah, it's one of the few commands that work differently on MS-Windows, because the shell command also works differently. Can be unexpected.

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