fixed root path error for windows

[zcodes]

there are two problems when change cwd to windows disk root like 'd:\'

1. if set 'noshellslash', nerdtree can open, but will crash when open sub directory of 'd:\', such as 'd:\docs'
2. if set 'shellslash', nerdtree can be opened, and error message will echo NERDTree: No bookmark or dirctory found for:

the core point of these problems is that the function getcwd() result diffrent style for 'd:\' and 'd:\docs',
if set 'noshellslash', getcwd() return 'd:\' with trailing slash and 'd:\docs' without trailing slash.
if set 'shellslash', getcwd() return 'd:/' and 'd:/docs', the function s:Path.AbsolutePathFor() does not handle
these differences properly.

this pr maybe related to #860 #890 #895

[PhilRunninger]

@zcodes, Thanks for the contribution. I have no way to verify the proposed changes for about a week. In the meantime, I have some comments on a couple lines of code.

[PhilRunninger]

Dang! FastHub (my phone app for GitHub) won't let me make comments on specific lines, so... Will line 28 work if you use s:Path.Slash() and eliminate the OR condition? If so, I think you don't even need the runningWindows() check anymore.

[PhilRunninger]

I believe the change to line 18 is not needed. That line is checking for the presence of one of the following: C:\, C:/, \\, or // at the beginning of the line.

[zcodes]

for the line 28, I simplify the code.

for line 18, the change is needed, some condition, the args (the path string) passed to the function is just 'd:', not 'd:\' or 'd:/', the result string will be 'd:d:/', that's why #860