add git diff check

[CagtayFabry]

Changes

• order the manifest contents outputs of devtools/scripts/update_manifest.py by filename for consistent manifest generation
• add a git diff check in pytest_asdf.yml workflow against the output of devtools/scripts/update_manifest.py (if this fails, try running devtools/scripts/update_manifest.py locally and commit changes)

[codecov]

Codecov Report

Merging #730 (ecfeb3c) into master (228554b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #730   +/-   ##
=======================================
  Coverage   95.88%   95.88%           
=======================================
  Files          92       92           
  Lines        6496     6496           
=======================================
  Hits         6229     6229           
  Misses        267      267           

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[github-actions]

Unit Test Results

       1 files  ±0         1 suites  ±0   3m 5s ⏱️ -31s
2 158 tests ±0  2 158 ✔️ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit ecfeb3c. ± Comparison against base commit 228554b.

♻️ This comment has been updated with latest results.

[CagtayFabry]

This should make it easier to find bugs in the asdf manifest and Converter setups

If the action fails at the git diff step run the update_manifest.py script (must be executed in the devtools/scripts folder) to fix @marscher @vhirtham