added explanation for "no new changes" error

Added a documentation page that explains the "no
new changes" error in details and what can be
done to solve the problem.

Signed-off-by: Edwin Kempin <edwin.kempin@gmail.com>
Change-Id: I3e979a0737408343fd4cfc31eec18e14526e6a08

Documentation/error-messages.txt

@@ -12,6 +12,7 @@ Error Messages
 * 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-new-changes.html[no new changes]
 * link:error-squash-commits-first.html[squash commits first]
 * link:error-you-are-not-author.html[you are not author ...]
 * link:error-you-are-not-committer.html[you are not committer ...]

Documentation/error-no-new-changes.txt

@@ -0,0 +1,45 @@
+no new changes
+==============
+
+With this error message Gerrit rejects to push a commit if the pushed
+commit was already successfully pushed to Gerrit. In this case there
+is no new change and consequently there is nothing to do for Gerrit.
+
+If your push is failing with this error message, you normally
+don't have to do anything since the commit was already successfully
+pushed. Still this error message may sometimes come as a surprise if
+you expected a new commit to be pushed. In this case you should
+verify that:
+1. your changes were successfully committed locally (otherwise there
+   is no new commit which can be pushed)
+2. you are pushing the correct commit (e.g. if you are pushing HEAD
+   make sure you have locally checked out the correct branch)
+
+If you are sure you are pushing the correct commit and you are still
+getting the "no new changes" error unexpectedly you can take the
+commit ID and search for the corresponding change in Gerrit. To do
+this simply paste the commit ID in the Gerrit WebUI into the search
+field. Details about how to search in Gerrit are explained link:user-search.html[here].
+
+Please note that each commit can really be pushed only once. This
+means:
+1. you cannot push a commit again even if the change for which the
+   commit was pushed before was abandoned (but you may restore the
+   abandoned change)
+2. you cannot reset a change to an old patch set by pushing the old
+   commit for this change again
+3. if a commit was pushed to one branch you cannot push this commit
+   to another branch
+
+If you need to re-push a commit you may rewrite this commit by
+link:http://www.kernel.org/pub/software/scm/git/docs/git-commit.html[amending] it or doing an interactive link:http://www.kernel.org/pub/software/scm/git/docs/git-rebase.html[git rebase]. By rewriting the
+commit you actually create a new commit (with a new commit ID) which
+can then be pushed to Gerrit. If the old commit contains a Change-Id
+in the commit message you also need to replace it with a new
+Change-Id (case 1. and 3. above), otherwise the push will fail with
+another error message.
+
+
+GERRIT
+------
+Part of link:error-messages.html[Gerrit Error Messages]