Ensure dir change removed despite exceptions

[code-tree]

Hi, so I don't believe the current implementation of cd() handles exceptions. Take for example:

try:
    with c.cd('subdir'):
        c.run('ls')
        raise Exception()
finally:
    c.run('ls')

The final run() call is executed in the same dir as the first because the dir change wasn't popped.

Note: Please test before merging (I'm writing in github to save time)

[bitprophet]

Good catch, thanks! Throwing in bugfix milestone, I'll handle backporting/testing when I get to it.

[bitprophet]

Looks like #640 extends this to handle the same issue in the sibling method, cwd, linking here for posterity. Merging these two now and adding tests/changelog/etc (😩)