Merge branch 'jc/stash-drop'

"git stash drop" is reimplemented as an internal call to
reflog_delete() function, instead of invoking "git reflog delete"
via run_command() API.

* jc/stash-drop:
  stash: call reflog_delete() in reflog.c
  reflog: libify delete reflog function and helpers
  stash: add tests to ensure reflog --rewrite --updatref behavior

diff --cc Makefile
Simple merge

diff --cc builtin/reflog.c
Simple merge

diff --cc builtin/stash.c
Simple merge

diff --cc t/t3903-stash.sh
Simple merge