* lisp/gnus/gnus-sum.el (gnus-tmp-thread): New var

The recent change to use lexical-binding introduced a bug because
gnus-summary-line-format-alist refers to variable `thread` which is now
lexical and hence not available there any more.  Replace it with
a dynamically scoped var `gnus-tmp-thread` as was already done with
all other vars.

(gnus-summary-line-format-alist): Use it instead of `thread`.
(gnus-summary-prepare-threads): Bind it around call to (eval
gnus-summary-line-format-spec).
1 job for master in 52 minutes and 12 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #1804
test-all

00:52:12

 
Name Stage Failure
failed
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
make[2]: *** [check-doit] Error 1
make[1]: *** [check-expensive] Error 2
Makefile:293: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:957: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1