Use JGit 5.1.15.202012011955-r javadoc to resume release build

The javadoc site for 5.1.16 is not available anymore on the
Eclipse's archive site: use the earlier version to resume the
ability to release Gerrit.

Change-Id: Ib1edf0cdefbed16a87a3187b33c0c4396a89e16f
diff --git a/lib/jgit/jgit.bzl b/lib/jgit/jgit.bzl
index 6ee9c29..9714767 100644
--- a/lib/jgit/jgit.bzl
+++ b/lib/jgit/jgit.bzl
@@ -2,7 +2,11 @@
 
 _JGIT_VERS = "5.1.16.202106041830-r"
 
-_DOC_VERS = _JGIT_VERS  # Set to _JGIT_VERS unless using a snapshot
+_DOC_VERS = "5.1.15.202012011955-r"  # Set to _JGIT_VERS unless using a snapshot
+# TODO: workaround to be removed when merging upstream:
+# 5.1.16.202106041830-r has been removed while
+# 5.1.15.202012011955-r is available and has the same
+# interface and docs
 
 JGIT_DOC_URL = "https://archive.eclipse.org/jgit/site/" + _DOC_VERS + "/apidocs"