The bug when introduced in OTP-19.0.

Optimize adding multiple modules to an Xref server

Adapt tag completion for changes in Emacs 25.

Stop storing module:tag entries in the completion table for definitions
in hrl-files.  These entries make no sense since hrl-files are not
modules.

Add test-case for tag completion in erlang-test.el.

Ensure prog-mode is defined for the sake of Emacs 23 and XEmacs.

Extracting data from debug info in BEAM files is parallelized.

The speed-up is not outstanding. If memory is not an issue, adding
`{spawn_opt, [{min_heap_size, <large number>}]}' to the option list of
xref:start/2 can make a (small) difference.

Reading of modules from a directory is parallelized. Attempts to
parallelize all modules when adding a release have been rejected since
the gain showed to be small compared to the increased complexity of
the code.

Using a process for coordinating the reading has not been attempted.

This reverts commit e020f75c10410a6943cd055bfa072a2641eab7da.

Add current module to non external functions calls.
Also improves finding record and macro definitions.

The right order is defined by find-tag-tag-order which is a list of
predicate functions.  Tags matched by the first function shall be
visited first.  After that tags matched by the seconds function shall be
visited and so on.

Previous behavior was to visit tags that match any of the functions in
the order they appear in the TAGS file.

Reported by Dan Gudmundsson.