make doesn't work when not using the git repository

As [reported by Ronnie Ridpath](http://librelist.com/browser//git.hub/2013/11/5/re-git-hub-version-0-5-release-candidate-2-released/):

```
[rridpath@Ridpath git-hub-0.5rc2]$ make
sed 's/^:Version: devel$/:Version: '"`git describe`"'/' man.rst | \
                rst2man --exit-status=1 > git-hub.1 || (rm -f git-hub.1 && false)
fatal: Not a git repository (or any parent up to mount point /home)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
<stdin>:11: (WARNING/2) Cannot extract empty bibliographic field "Version".
make: *** [git-hub.1] Error 1

A quick diff between this version and the 0.4.1 version that worked for me shows
that you've changed this to git describe which doesn't work if you're not in a git
repo which I won't be if I've just untarred

21c20
<       sed 's/^:Version: devel$$/:Version: $(version)/' $< | \
---
>       sed 's/^:Version: devel$$/:Version: '"`git describe`"'/' $< | \
30c29
```