Simplify naming (versioning) of non-release tig tarball and rpm file

Simplify naming (versioning) of non-release tig tarball and rpm file

Use the tag part of git-describe output, with `tig-' prefix stripped
off, as version number. Use number of commits since given tag as
release number, using 0 if we are at tagged version, and adding
'.dirty' suffix if working directory is dirty. Do not use 'g<sha 1>'
part of git-describe output.

For example, when git-describe output was tig-0.7-8-g26d0386, before
this commit tarball was named tig-0.7.8.g26d0386.tar.gz and rpm was
named tig-0.7.8.g26d0386-1.src.rpm (for dirty tree tarball was
tig-0.7.8.g26d0386.dirty.tar.gz), now tarball is named
tig-0.7-8.tar.gz and rpm is named tig-0.7-8.src.rpm (for dirty tree
tarball is tig-0.7-8.dirty.tar.gz).

[ Simplify the version extraction a bit. --jonas ]

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Jonas Fonseca <fonseca@diku.dk>