Added explanation for "no common ancestry" error

Added a documentation page that explains the
"no common ancestry" error.

Change-Id: I89335d9491413e1b42679024950307bb466f440f
Signed-off-by: Edwin Kempin <edwin.kempin@gmail.com>
diff --git a/Documentation/error-messages.txt b/Documentation/error-messages.txt
index 2f0a8f2..e84e6b3 100644
--- a/Documentation/error-messages.txt
+++ b/Documentation/error-messages.txt
@@ -14,6 +14,7 @@
 * link:error-invalid-changeid.html[invalid Change-Id line format in commit message]
 * link:error-missing-changeid.html[missing Change-Id in commit message]
 * link:error-multiple-changeid-lines.html[multiple Change-Id lines in commit message]
+* link:error-no-common-ancestry.html[no common ancestry]
 * link:error-no-new-changes.html[no new changes]
 * link:error-not-a-gerrit-project.html[not a Gerrit project]
 * link:error-not-valid-ref.html[not valid ref]