record: use absolute path instead of os.chdir

Record was changing the current directory to `repo.root` in order to be able to
feed `command.commit` file name relative to this `repo.root`. This is a bit
overkill and prevent an incoming fix to rebase. This would also break
multi-threaded usage.

Instead we just feed `command.commit` with absolute path name. works as well as
before but without chdir.

--HG--
branch : stable
rebase: do not crash in panic when cwd disapear in the process (issue4121)

Before this patch rebase crashed badly when it happend. (not abort, crash).

Fix courtesy of Matt Mackall.

--HG--
branch : stable