ReceiveCommits: uniformize commit validation error messages.

Before

  [abcdabc] missing subject; Change-Id must be in commit message
  footer

After

  commit abcdabc: missing subject; Change-Id must be in message
  footer

Change-Id: I15ca67fb591b0d237cb9217936853ec8dc706aaa
diff --git a/Documentation/error-missing-changeid.txt b/Documentation/error-missing-changeid.txt
index 9cddd85..08f2c09 100644
--- a/Documentation/error-missing-changeid.txt
+++ b/Documentation/error-missing-changeid.txt
@@ -1,4 +1,4 @@
-= missing Change-Id in commit message footer
+= commit xxxxxxx: missing Change-Id in message footer
 
 With this error message Gerrit rejects to push a commit to a project
 which is configured to always require a Change-Id in the commit
diff --git a/Documentation/error-missing-subject.txt b/Documentation/error-missing-subject.txt
index 3703ade..6ef37a4 100644
--- a/Documentation/error-missing-subject.txt
+++ b/Documentation/error-missing-subject.txt
@@ -1,4 +1,4 @@
-= missing subject; Change-Id must be in commit message footer
+= commit xxxxxxx: missing subject; Change-Id must be in message footer
 
 With this error message Gerrit rejects to push a commit to a project
 which is configured to always require a Change-Id in the commit
diff --git a/Documentation/error-multiple-changeid-lines.txt b/Documentation/error-multiple-changeid-lines.txt
index 0729547..31567f4 100644
--- a/Documentation/error-multiple-changeid-lines.txt
+++ b/Documentation/error-multiple-changeid-lines.txt
@@ -1,4 +1,4 @@
-= multiple Change-Id lines in commit message footer
+= commit xxxxxxx: multiple Change-Id lines in message footer
 
 With this error message Gerrit rejects to push a commit if the commit
 message footer of the pushed commit contains several Change-Id lines.