If there is just one repository, always use it as defaultRepository

Beyond making "branches" and filter part of "home" pages more convenient to
use for newly setup single repository Critic instances, this also makes
behavior consistent with what "Config" displays for such an instance.
Although the user had no preference recorded for a newly installed
instance, the "Config" page was populating the defaultRepository dropdown
with a single item representing the only existing repository so for the
user it looked like they had the "Config" properly setup. Just hitting
"Save" without changing anything did indeed set the only repository as
defaultRepository, but such a click should not be necessary ofc.

dbutils.py

@@ -249,7 +249,17 @@ def setPreference(self, db, item, value):
                 cursor.execute("INSERT INTO userpreferences (uid, item, string) VALUES (%s, %s, %s)", [self.id, item, str(value)])
 
     def getDefaultRepository(self, db):
-        return gitutils.Repository.fromName(db, self.getPreference(db, "defaultRepository"))
+        default_repo = self.getPreference(db, "defaultRepository")
+        if not default_repo:
+            cursor = db.cursor()
+            cursor.execute("SELECT COUNT(*) FROM repositories")
+
+            repo_count = cursor.fetchone()[0]
+            if repo_count == 1:
+                cursor.execute("SELECT name FROM repositories")
+                default_repo = cursor.fetchone()[0]
+
+        return gitutils.Repository.fromName(db, default_repo)
 
     def getResource(self, db, name):
         if name in self.__resources: