Use a tempfile instead of building our own.

[tenderlove]

This commit changes cache writing to use the Tempfile class rather than
constructing our own tempfile. It means we don't have to deal with
unlinking the tempfile in case there's an exception, and it also fixes
this code on Ruby 2.5 (Dir::Tmpname.make_tmpname is no longer
available)

Before this change:

[aaron@TC linguist (master)]$ ruby -I lib:~/git/charlock_holmes/lib:~/github/rugged/lib bin/git-linguist -c 15885701cd883b633847979583f128e4ba1beac9 breakdown
{"Ruby":["Gemfile","Rakefile","bin/git-linguist","bin/linguist","github-linguist.gemspec","lib/linguist.rb","lib/linguist/blob.rb","lib/linguist/blob_helper.rb","lib/linguist/classifier.rb","lib/linguist/file_blob.rb","lib/linguist/generated.rb","lib/linguist/grammars.rb","lib/linguist/heuristics.rb","lib/linguist/language.rb","lib/linguist/lazy_blob.rb","lib/linguist/md5.rb","lib/linguist/repository.rb","lib/linguist/samples.rb","lib/linguist/shebang.rb","lib/linguist/strategy/extension.rb","lib/linguist/strategy/filename.rb","lib/linguist/strategy/modeline.rb","lib/linguist/tokenizer.rb","lib/linguist/version.rb","script/add-grammar","script/convert-grammars","script/fast-submodule-update","script/licensed","script/list-grammars","script/prune-grammars","script/set-language-ids","test/helper.rb","test/test_blob.rb","test/test_classifier.rb","test/test_color_proximity.rb","test/test_file_blob.rb","test/test_generated.rb","test/test_grammars.rb","test/test_heuristics.rb","test/test_instrumentation.rb","test/test_language.rb","test/test_md5.rb","test/test_modelines.rb","test/test_pedantic.rb","test/test_repository.rb","test/test_samples.rb","test/test_shebang.rb","test/test_tokenizer.rb"],"Shell":["script/bootstrap","script/cibuild","script/travis/before_install"]}
Traceback (most recent call last):
	4: from bin/git-linguist:150:in `<main>'
	3: from bin/git-linguist:135:in `git_linguist'
	2: from bin/git-linguist:37:in `linguist'
	1: from bin/git-linguist:50:in `save_language_stats'
bin/git-linguist:80:in `write_cache': uninitialized constant GitLinguist::Tempfile (NameError)

After this change:

[aaron@TC linguist (ruby-2.5)]$ ruby -I lib:~/git/charlock_holmes/lib:~/github/rugged/lib bin/git-linguist -c 15885701cd883b633847979583f128e4ba1beac9 breakdown
{"Ruby":["Gemfile","Rakefile","bin/git-linguist","bin/linguist","github-linguist.gemspec","lib/linguist.rb","lib/linguist/blob.rb","lib/linguist/blob_helper.rb","lib/linguist/classifier.rb","lib/linguist/file_blob.rb","lib/linguist/generated.rb","lib/linguist/grammars.rb","lib/linguist/heuristics.rb","lib/linguist/language.rb","lib/linguist/lazy_blob.rb","lib/linguist/md5.rb","lib/linguist/repository.rb","lib/linguist/samples.rb","lib/linguist/shebang.rb","lib/linguist/strategy/extension.rb","lib/linguist/strategy/filename.rb","lib/linguist/strategy/modeline.rb","lib/linguist/tokenizer.rb","lib/linguist/version.rb","script/add-grammar","script/convert-grammars","script/fast-submodule-update","script/licensed","script/list-grammars","script/prune-grammars","script/set-language-ids","test/helper.rb","test/test_blob.rb","test/test_classifier.rb","test/test_color_proximity.rb","test/test_file_blob.rb","test/test_generated.rb","test/test_grammars.rb","test/test_heuristics.rb","test/test_instrumentation.rb","test/test_language.rb","test/test_md5.rb","test/test_modelines.rb","test/test_pedantic.rb","test/test_repository.rb","test/test_samples.rb","test/test_shebang.rb","test/test_tokenizer.rb"],"Shell":["script/bootstrap","script/cibuild","script/travis/before_install"]}
[aaron@TC linguist (ruby-2.5)]$

[lildude]

LGTM. Thanks @tenderlove. Merge when ready. Lemme know when you want this on .com and I'll make a new release (tomorrow 🇬🇧 time at the earliest).

[tenderlove]

@lildude thank you! Whenever you have time. It isn't urgent, but I can't upgrade us to Ruby 2.5 without this. Thanks!

[kivikakk]

Going to do a release now so we can see what the 2.5 PR looks like!

[kivikakk]

Unfortunately this fails in some cases since we try to File.rename the created tempfile across mounts. It uses rename(2), which fails with EXDEV.

Dir::Tmpname.make_tmpname takes basename as the first argument, which can include a directory — it's always the prefix of the generated filename. (No directory in basename = no directory in resulting path.) Instead we're now always dropping them in the global temp dir, which you could reasonably expect to be on a different mount (/tmp).

We'll need to change this to drop it in the same directory like we were before.