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.

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.

help: improve description of phases.checksubrepos setting The existing description was a bit hard to understand.