Make gitrev more reliable

Make gitrev more reliable

In some build methods, the gitrev is unnecessarily set to "unknown".
We can improve this by changing the gitrev to use
`git describe --always --long --dirty`.

This gets the revision even when no tag matches (--always).  It prints
the hash even when it exactly matches a tag (--long).  And if there are
uncommitted changes, it appends "-dirty", rather than failing (--dirty).

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Thode <prometheanfire@gentoo.org>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #8034