Merge branch 'rs/pretty-add-again' into maint

The pretty-format specifiers like '%h', '%t', etc. had an
optimization that no longer works correctly.  In preparation/hope
of getting it correctly implemented, first discard the optimization
that is broken.

* rs/pretty-add-again:
  pretty: recalculate duplicate short hashes

diff --cc pretty.c
Simple merge

diff --cc strbuf.c
Simple merge

diff --cc strbuf.h
Simple merge