experiments: make dvc exp show more consistent w/viewer

[pmrowla]

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

• Show git tag/branch names instead of SHAs where applicable
  • SHAs can still be displayed using dvc exp show --sha ...
• Make timestamp formatting less verbose

❯ dvc exp show --all-commits --include-params=featurize --no-pager
┏━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━┳━━━━━━━━━┳━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━┓
┃ Experiment            ┃ Created      ┃     auc ┃ featurize.m… ┃ featurize.ng… ┃
┡━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━╇━━━━━━━━━╇━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━┩
│ workspace             │ -            │ 0.50822 │ 1500         │ 4             │
│ executor-tree         │ 05:31 PM     │ 0.50822 │ 1500         │ 4             │
│ └── *41a9f4b          │ 05:34 PM     │       - │ 456          │ 5             │
│ 6d18fc1               │ Aug 24, 2020 │ 0.58164 │ 1500         │ 3             │
│ ├── 3c8db11           │ Aug 24, 2020 │ 0.50298 │ 500          │ 4             │
│ ├── 5840202           │ Aug 24, 2020 │ 0.50822 │ 1500         │ 4             │
│ └── *3b6e442          │ Aug 25, 2020 │       - │ 123          │ 5             │
│ 0562956               │ Aug 24, 2020 │ 0.61314 │ 1500         │ 2             │
│ ├── e6d1716           │ Aug 24, 2020 │ 0.58164 │ 1500         │ 3             │
│ └── *c34fd7e          │ Aug 25, 2020 │       - │ 456          │ 5             │
│ acd59eb               │ Jul 22, 2020 │ 0.61314 │ 1500         │ 2             │
│ 11-bigrams-experiment │ Jun 20, 2020 │ 0.61314 │ 1500         │ 2             │
│ 10-bigrams-model      │ Jun 20, 2020 │ 0.54175 │ 1500         │ 2             │
...
│ 0-git-init            │ Jun 20, 2020 │       - │ 1500         │ 4             │
└───────────────────────┴──────────────┴─────────┴──────────────┴───────────────┘