Improve reject message if Change-Id is in subject

Some users that are new to Git and Gerrit manage to create commits
without subject and message, but with a Change-Id footer. This happens
mainly from EGit where the Change-Id footer is inserted automatically
and users just confirm the commit dialog without typing any subject or
message. This results in a commit message that looks like this:
"
Change-Id: I4bcf4f0bd8ed8da41ce53efd7298b80cec492d64
"

Pushing this change for review if Change-Id is required results in the
following error message:

"remote: ERROR: missing Change-Id in commit message footer
remote: Suggestion for commit message:
remote:
remote: Change-Id: I4bcf4f0bd8ed8da41ce53efd7298b80cec492d64"

This confuses the users since
a) they have a Change-Id in the commit message and
b) the suggested commit message looks like the existing commit message.

Handle this special case and provide a specific error message:
"missing subject, Change-Id must be in commit message footer"

Change-Id: I4bcf4f0bd8ed8da41ce53efd7298b80cec492d64
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
4 files changed