move the `update-reference.sh` script into `x.py`

[nikomatsakis]

The script that updates reference files for UI tests is currently a shell script, which has portability issues. It is also a bit annoying to use since you must specify the path to your build directory and so forth. It'd be nice to fix all these problems and move it into x.py.

[nikomatsakis]

I think I would like it to work like this:

• ./x.py bless
  • Updates all references through the test suite.
  • Rather like update-all-references.sh today
• ./x.py bless src/test/ui/foo*.rs
  • Updates the references for some particular test(s).
  • Rather like update-reference.sh today
• Maybe some way to specify the target, revision?
  • This seems less important.

[nikomatsakis]

cc @petrochenkov, who may have opinions on this topic.

[nikomatsakis]

cc @ehuss, who mentioned being up for doing the leg work =)

[petrochenkov]

Despite working on Windows, update-all-references being a shell script never bothered me due to MSYS2 (or Git Bash), but I agree that integrating it into x.py will simplify its use, especially the first use.

I also never used update-reference.sh personally, only update-all-references.sh. The latter works quite fast so I'm not sure why the former is necessary.