Check remote branch existence on sourcegraph remote, not origin

[logan]

My team has a particular setup that currently isn't supported by this plugin. For various reasons, the HEAD branch of our sourcegraph remote is not the same as the HEAD branch of our origin. On origin, we use develop, and on sourcegraph we use master. If I'm on develop and try to open a link to Sourcegraph, the plugin confirms that develop exists on origin, but not that it exists on sourcegraph.

sourcegraph-jetbrains/src/Util.java

Lines 101 to 104 in c73b380

	// If on a branch that does not exist on the remote, use "master" instead.
	if (!isRemoteBranch(branch, repoRoot)) {
	branch = "master";
	}

There are two assumptions in the above snippet that are not completely general:

1. The remote branch that matters may be sourcegraph instead of origin (which is hard-coded in isRemoteBranch).

2. The HEAD for the remote branch that matters may not be master.

A fix for (1) solves for my team's specific circumstances. A fix for (2) would make this plugin more robust, but isn't as high of a priority.

I'm not sure the best way to implement the fix for (2). If you run git remote show sourcegraph you could grep for a line that reads HEAD branch: master. Maybe there's a better approach? Alternatively, add a simple branch setting to the .idea/sourcegraph.xml config file to override the assumption of master.