Add a few formatting directives to "git for-each-ref --format=...",
to paint them in color, etc.
* rr/for-each-ref-decoration:
for-each-ref: avoid color leakage
for-each-ref: introduce %(color:...) for color
for-each-ref: introduce %(upstream:track[short])
for-each-ref: introduce %(HEAD) asterisk marker
t6300 (for-each-ref): don't hardcode SHA-1 hexes
t6300 (for-each-ref): clearly demarcate setup