Resetting a check out in Super Quick Patch Guide

Add advice so that somebody wishing to submit a second patch doesn't need
to throw away their perl check-out and start again.

Not knowing the 'git clean' step caught me out, and meant perl wouldn't
build for me. Nicholas helped me out. Adding this to the guide will
hopefully save Nicholas from having to repeat that for others (especially
since others may not be fortunate enough to have Nicholas handily seated
next to them at the point they encounter it).

(The non-building was because some things in the repository had been
re-arranged since my previous patch (several months earlier), and the
latest build was getting confused by some files left over from a
pre-re-arragned build.)

The 'git clean' step will also remove the first 0001-*.patch file, avoiding
the problem of there being two files matching that glob when attaching the
second patch.

Committer: Removed trailing whitespace.

For: RT #119599