Re: Improve content of tree.gittxt (issue 564990043 by address@hidden)

[knupero]

Patchset 2 was reviewed, counted down, pushed to staging, made it to
master. Then David's patchy broke because I incorrectly assumed that
every clone of a lilypond repository has a local copy of the master
branch. Consequently the patch was reverted in staging and master.

The fix is obvious, we must use 'origin/master' instead of 'master'.

Please review and countdown again.

https://codereview.appspot.com/564990043/