Add log for error in git mirror (#1689)

Without this log you will get no trace of the error,
only the message of it in the mirror log. This may
not be enough to trace the cause of the error.

scm-plugins/scm-git-plugin/src/main/java/sonia/scm/repository/spi/GitMirrorCommand.java

@@ -151,6 +151,7 @@ public class GitMirrorCommand extends AbstractGitCommand implements MirrorComman
         return doUpdate();
       } catch (GitAPIException e) {
         result = FAILED;
+        LOG.info("got exception while trying to synchronize mirror for repository {}", context.getRepository(), e);
         mirrorLog.add("failed to synchronize: " + e.getMessage());
         return new MirrorCommandResult(FAILED, mirrorLog, stopwatch.stop().elapsed());
       }