Show names/sha1 of detected branches on failure

When the assertion fails, show the details of the set of branches so
that the test failure has more information about the failure conditions.

Preparation for a future time when a failure like this recurs:

* jenkinsci/bom#4193

Thanks to @darinpope for the question that prompted the improvement

src/test/java/org/jenkinsci/plugins/gitclient/GitAPITest.java

@@ -760,7 +760,7 @@ public void testListRemoteBranches() throws Exception {
         workspace.launchCommand("git", "fetch", "origin");
         Set<Branch> branches = testGitClient.getRemoteBranches();
         assertBranchesExist(branches, "origin/" + defaultBranchName, "origin/test", "origin/another");
-        assertEquals(3, branches.size());
+        assertEquals("Wrong branch count, found " + branches.size() + " branches: " + branches, 3, branches.size());
     }
 
     @Test