Fix tag format of container when building tag

[jwliechty]

Another PR that addresses #63.

Doing a captain build when on a tagged commit currently causes a
erroneous tag name. For example, if on a tag v1.0 of a repository, when
the captain build occurs, this this the output:

[CAPTAIN] Skipping build of image my_organization/my_image_name:da5ewb3 - image is already built
[CAPTAIN] Tagging image my_organization/my_image_name:da5ewb3 as my_organization/my_image_name:latest
[CAPTAIN] Tagging image my_organization/my_image_name:da5ewb3 as my_organization/my_image_name:tags.v1.0^0
API error (500): {"message":"invalid tag format"}

Notice the tags.v1.0^0. That is wrong on two fronts. 1.) the tag should
be v1.0 and 2.) the carrot symbol ^ makes it an invalid format for a
docker tag.

The problem occurs when obtaining the branch names. The code uses the
command git name-rev --name-only HEAD to determine the branch name.
This used to work. However, git must have changed over the years because
if you are on a tag, this now returns tags.v1.0^0.

Fortunately, the same command has an --exclude parameter that will
allow us to indicate to filter out tags (which is what we wanted anyway
for that particular call). The tags are determined later via the git
command git tag --points-at HEAD. (all of these references occur in
git.go)

When running tests, as they currently are, they failed for this very
reason. After excluding tags with the --exclude parameter to the git
command, all the tests pass. (I am running git version 2.17.1 on Ubuntu
18.04)

Please accept this PR and create a new distributable so I and others do
not run into this again.

[CLAassistant]

All committers have signed the CLA.

[dkapanidis]

Tested and it works properly.

Here is a quick way to test it locally (a variation of the test defined at #63)

git init

cat > Dockerfile << EOL
FROM scratch
EXPOSE 80
EOL

git add Dockerfile
git commit -am 'Testing'
git tag -a 1.0.0 -m 'test tag'
captain build