Use format +git:XXXXXXXX

The colon is kinder to Debian than a dash and seems stylistically
better.  The dash feels as though it introduces another version level,
e.g. -rc1-alpha2.  We could just do +gitXXXXXXXX but as git commit
hashes are in lowercase hexadecimal the result can be something really
ugly like +giteaf0cb71.  It's also easier to select the commit hash
for a copy/paste operation when there is something to separate it.