Hello emacs-devel,

"vc-git-mode-line-string" (which is called automatically) failed
occasionally with an error message (wrong-type-argument arrayp nil) when
the default-directory is outside the repository.

I hunted the error down to "vc-working-revision" function, which
returned nil under those particular circumstances. The returned value
(nil) was then passed to as an argument to "substring", causing a type
error.

If there are better ways to resolve this issue, I would be very happy to
discover them. Nonetheless I've seen such approach of "let"-ing the
default-directory variable a few times across the emacs source code
tree.

Would doing this on a lower level, for example in "vc-call-backend" be a
better approach?