Keep current alternate-file name while reading templates

[cakturk]

vim-go doesn't keep the previous value of alternate-file register
while reading templates. As a result, you will not be able to go back
to the buffer you were editing.

Suppose you have a directory structure like:

.
├── cmd/
│   └── bar/
└── foo.go

And you are editing a file in vim:

$ vim foo.go

and then you decided to create a new go file like:

:e cmd/bar/bar.go

Once you finished with 'bar.go', you hit CTRL-^ to return to the
previously edited file (foo.go), but you unexpectedly end up in
$RTP/vim-go/templates/hello_world.go.

This PR addresses this issue by prefixing the :read command in
go#template#create() with :keepalt.

[codecov-io]

Codecov Report

Merging #1675 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1675   +/-   ##
=======================================
  Coverage   22.36%   22.36%           
=======================================
  Files          53       53           
  Lines        4226     4226           
=======================================
  Hits          945      945           
  Misses       3281     3281

Flag	Coverage Δ
#nvim	16.91% <100%> (ø)	⬆️
#vim74	19.4% <100%> (ø)	⬆️
#vim80	21.15% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
autoload/go/template.vim	64.7% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cc0467c...76775e0. Read the comment docs.

[arp242]

Thanks!