scheduler: resolve git references into revisions on submission

[charlesbaynham]

ARTIQ Pull Request

Description of Changes

Allow users to submit ARTIQ experiments with non-hash revisions.

Currently, the revision field for expid can be blank (results in the current HEAD) or set to a fully resolved git hash. This PR allows the user to use e.g. a branch or tag instead (like "my-working-branch"). The reference is expanded into a full git hash as usual for expid, so ends up in the HDF5 datasets etc like normal.

Related Issue

#1543

Type of Changes

	Type
✓	✨ New feature

Steps (Choose relevant, delete irrelevant before submitting)

All Pull Requests

• Use correct spelling and grammar.
• Update RELEASE_NOTES.rst if there are noteworthy changes, especially if there are changes to existing APIs.
• Close/update issues.
• Check the copyright situation of your changes and sign off your patches (git commit --signoff, see copyright).

Code Changes

• Run flake8 to check code style (follow PEP-8 style). flake8 has issues with parsing Migen/gateware code, ignore as necessary.
• Test your changes or have someone test them. Mention what was tested and how.
• Add and check docstrings and comments
• Check, test, and update the unittests in /artiq/test/ or gateware simulations in /artiq/gateware/test

Documentation Changes

• Check, test, and update the documentation in doc/. Build documentation (cd doc/manual/; make html) to ensure no errors.

Git Logistics

• Split your contribution into logically separate changes (git rebase --interactive). Merge/squash/fixup commits that just fix or amend previous commits. Remove unintended changes & cleanup. See tutorial.
• Write short & meaningful commit messages. Review each commit for messages (git show). Format:

  topic: description. < 50 characters total.
  
  Longer description. < 70 characters per line
  

Licensing

See copyright & licensing for more info.
ARTIQ files that do not contain a license header are copyrighted by M-Labs Limited and are licensed under LGPLv3+.

[charlesbaynham]

I haven't found a canonical term for something that can be either a reference or a revision, so I've just written it out here. Another option would be "refish" which is used by the libgit2 library:

https://github.com/libgit2/libgit2/blob/main/examples/common.c#L105C5-L116

...but not elsewhere.

[charlesbaynham]

It could also be "refish" in the GUI, but I think it's better to be explicit. Though I shortened "Revision" to "Rev" to avoid having the box take up too much space...

[charlesbaynham]

@sbourdeauducq I've resolved conflicts with the latest master - would it be possible to merge this soon, before others arise? I think all the changes we've discussed are in.