GitUtils

class GitUtils {

    def final static NO_BRANCH = new Branch(name: "NO_BRANCH")
    def final static NO_TAG = new Tag(name: "NO_TAG")
    
    
    static class Tag implements CharSequence {
        private @Delegate CharSequence name
        
        @Override
        String toString() { name ? name.toString() : 'null' }
        
        def hasPrefix = { name && name.startsWith(it) }
    
    }

    static class Branch implements CharSequence {
        private @Delegate CharSequence name
        
        @Lazy def isCurrent = name && length() > 0 && charAt(0) == '*'
        
        @Override
        public String toString() { isCurrent ? name.toString().substring(2) : name.toString().trim() }

    }
    
    private class TagList extends List<Tag> {
        @Delegate List<Tag> tags = execute("tag").collect { new Tag(name: it) } ?: [NO_TAG]
        def by = { prefix -> tags.findAll{ it.hasPrefix(prefix) } ?: [NO_TAG] }
    }
    
    private class BranchList extends List<Branch> {
        @Delegate List<Branch> branches = execute("branch").collect { new Branch(name: it) } ?: [NO_BRANCH]
        @Lazy def current = branches?.find { it.isCurrent } ?: NO_BRANCH
    }

    @Lazy def tags = new TagList();
    @Lazy def branches = new BranchList();

    /**
     * Path in which to work on
     */
    def final path

    GitUtils(File path) {
        this.path = path
    }

    /**
     * Executes the git command and returns an array with all the
     */
    def execute = { command ->
        def proc = ("git " + command).execute(null, path)
        proc.waitFor()
        def out = proc.in.text.trim()
        // return null if no output is provided to use the elvis format
        out && out.length() > 0 ? out.split("\n") : null
    }
    
}

Usage:

def utils = new GitUtils(new File("/git/epo/"))
utils.tags.by('b')
utils.branches.current