Auto merge of #1164 - alexcrichton:issue-209, r=huonw

Auto merge of #1164 - alexcrichton:issue-209, r=huonw

All paths printed will now be absolute paths unless the path is a descendant of
the current directory. This should keep error messages and warnings of a
reasonable length when working with the local project while still allowing
errors in registry/git dependencies to be tracked down.

Special care is taken in these situations to ensure that the error message from
the compiler prints a reasonable path.

Closes #209
Closes #694