Save machines from extra work

After committing small changes, BuildBot and TestBot do unnecessary work, building and testing what remains essentially the same thing.

I propose the following rules.

1. Some special sign at the end of the first (alternatively, last) line of the commit message (not counting possible empty lines) indicates that TestBot should skip this revision.
Example:

[EXPLORER] *
Fix memory leak.

2. If the patch to be committed is trivial, use some other special sign to stop BuildBot from building.
Examples:

[SMSS]

• Replace tabs by spaces.
• Improve comments. **

[NTOSKRNL]
Add more parentheses to please Alex.
**

These special signs could also be helpful for people.