build.go: use new combined version string
build.go: use new combined version string

Previously, when a VERSION file exists it takes precendence over the
git version. This is unfortunate because all restic binaries compiled
from a git checkout will just identify as the latest release (e.g.
'0.1.0'), regardeless of any commits on top of it.

This commit adds a combined version string by using the contents of
the VERSION file, and append the current git version returned by `git
describe` if available, e.g.:

    0.1.0 (v0.1.0-6-gb188217-dirty).

arithmetric/restic,byo/restic,fawick/restic,episource/restic,middelink/restic,pombredanne/restic,fawick/restic,ckemper67/restic,jayme-github/restic,restic/restic,ar-jan/restic,byo/restic,howeyc/restic,bchapuis/restic,ar-jan/restic,stuertz/restic,stuertz/restic,jayme-github/restic,restic/restic,middelink/restic,kurin/restic,mappu/restic,kurin/restic