bug#19741: 25.0.50; find-tag completion uses an outdated cache of the tags table

[Dmitry Gutov]

1. M-x find-file lisp/progmodes/etags.el. Being in that buffer, and not
in *scratch*, is somehow important.

2. M-x visit-tags-table ../../lisp/TAGS.

3. M-x find-tag, press TAB, see the table loaded.

4. M-x visit-tags-table, ../../src/TAGS, press `y' (important!).

5. M-x find-tag, type `display_li', press TAB, see [No matches]!

The main scenario ends here.

6. Finish typing `display_line', press RET, see the navigation succeed
anyway (with a jump to src/xdisp.c:20013).

7. Press `M-,' to get back to etags.el, repeat 5., see the same result.

8. M-x find-file ../../src/search.c, repeat 5., see a different result.

In GNU Emacs 25.0.50.2 (x86_64-unknown-linux-gnu, GTK+ Version 3.12.2)
 of 2015-01-30 on axl
Repository revision: 9242cdcda95e0fcb57233a8665d251e280eddec6
Windowing system distributor `The X.Org Foundation', version 11.0.11601901
System Description:     Ubuntu 14.10