Add install from tag

[saizai]

Fixes #4312.

Changes proposed in this pull request:

• add --tag to rvm install
• add tag to ruby string

[pkuczynski]

@mpapis ?

[mpapis]

all is fine, but wouldn't it be better to fix rvm_ruby_tag?

[saizai]

Seems to be two totally different things - `rvm_ruby_tag` is basically the "version" of a ruby; `rvm_ruby_repo_tag` is tag in the `git` sense (i.e. a nickname for a particular commit). There's no guarantee that rubies will be stored in git repos or have git tags to mark their releases. Nor will all git tags be releases. I tried to make a minimal change so as to *not* mess with the `rvm_ruby_tag` code. :-P

[mpapis]

rvm_ruby_repo_tag="${tag_name}"

[mpapis]

how this is different from the earlier line __from="${rvm_ruby_tag#t}"? why can't we use that?

[saizai]

rvm_ruby_tag is the "name" of a release for display purposes. It may or may not be stored as a git tag in the repo. rvm_ruby_repo_tag (what I added) is specifically a git tag. It may or may not be a release.

[mpapis]

but they both end up in the __from variable which controls how we get the source code, I'm just having trouble understanding how it's different if finally they land in exactly the same variable.

[saizai]

__from gets used, here, as git checkout $__from. git checkout works equally well for tags, branches & commit IDs, so I just reused it.

In other parts, it makes a difference — e.g. the gem name & pulling the tag description from git. Or going around the tag-as-in-name lookup and just doing tag-as-in-git.