Convert empty git refs to 'HEAD' (Fixes #2992)

[tjd2002]

As discussed at #2992, if an empty git_rev line is included in a meta.yaml file, then conda-build may build from a stale git commit in the cache (rather than the cache being updated to the current HEAD).

This is because an empty line results in the Source_dict.git_rev key containing an empty dict, which does not get replaced with the default value ('HEAD') when returned with dict.get. Then that empty dict value causes get_mirror_checkout_recursive to skip the logic it has to update the local cache's HEAD (since it assumes any value other than 'HEAD' is a specific commit).

The two fixes here are currently redundant (either one would prevent the bug seen in #2992), but they both seem reasonable to me.

Thanks to @stuarteberg for suggestions.

[tjd2002]

The CI tests that fail here seem to be the same ones currently failing in master.

I'm not sure the best place for a regression test for this fix, however the following steps trigger the bug in conda-build 3.10.9 (with conda 4.6.0a2):

mkdir issue-2992 issue-2992-buildroot
cd issue-2992
git init
printf "package:\n  name: cb2992\nsource:\n  git_url: .\n  git_rev:\n" > meta.yaml
git add meta.yaml
git commit -m "commit #1 (on master)"
conda-build --croot ../issue2992-buildroot meta.yaml # builds from master #1
git commit -m "commit #2 (on master)"
conda-build --croot ../issue2992-buildroot meta.yaml # builds from master #2
git checkout -b "new-branch"
git commit --allow-empty -m "commit #3 (on new-branch)"
conda-build meta.yaml # builds from commit #2 <--ERROR: should be #3 on new-branch
# deleting git cache fixes
rm -rf ../issue2992-buildroot/git_cache
conda-build meta.yaml # builds from commit #3 <--FIXED; builds from new-branch

(It seems like conda_build will update if HEAD is on the same branch, but if you switch to another branch after the initial build it won't follow.)

[msarahan]

Thanks! Merging.

[github-actions]

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Thanks!