Remove branch name from push

It is possible to specify an alternative default initial branch for git
repositories. This is globally configurable with **init.defaultBranch**.
See `man git-init` for details.

We should be able to rely on pushing and pulling from the default
branch without explicitly setting "master", or anything else. dstask
will simply push and pull from the remote named "origin" using whatever
default tracking branch is set.

Refs naggie#116

git.go

@@ -110,6 +110,6 @@ func EnsureRepoExists(repoPath string) {
 // Sync performs a git pull, and then a git push. If any conflicts are encountered,
 // the user will need to resolve them.
 func Sync(repoPath string) {
-	MustRunGitCmd(repoPath, "pull", "--no-rebase", "--no-edit", "--commit", "origin", "master")
+	MustRunGitCmd(repoPath, "pull", "--no-rebase", "--no-edit", "--commit", "origin")
 	MustRunGitCmd(repoPath, "push", "origin", "master")
 }