Merge branch 'ab/bug-if-bug'

A new bug() and BUG_if_bug() API is introduced to make it easier to uniformly log "detect multiple bugs and abort in the end" pattern. * ab/bug-if-bug: cache-tree.c: use bug() and BUG_if_bug() receive-pack: use bug() and BUG_if_bug() parse-options.c: use optbug() instead of BUG() "opts" check parse-options.c: use new bug() API for optbug() usage.c: add a non-fatal bug() function to go with BUG() common-main.c: move non-trace2 exit() behavior out of trace2.c