Add `fetch commits` functionality in case of unknown commit

[memona008]

Resolves #254

Why does this pull request exists?

When there is new commits in git repository, cli wont fetch the latest commits and abort by declaring the commit unknown.

Changes made in PR:

• Fetch latest details when a commit is unknown
• Modified tests according to the case

[codecov[bot]]

Codecov Report

Merging #292 (5aa19d8) into master (550f7d3) will decrease coverage by 0.02%. The diff coverage is 81.48%.

@@            Coverage Diff             @@
##           master     #292      +/-   ##
==========================================
- Coverage   86.68%   86.67%   -0.02%     
==========================================
  Files         126      126              
  Lines        4289     4308      +19     
  Branches      886      894       +8     
==========================================
+ Hits         3718     3734      +16     
- Misses        390      393       +3     
  Partials      181      181              

Files	Coverage Δ
tests/test_utils_commits.py	100.00% <100.00%> (ø)
valohai_cli/api.py	93.42% <0.00%> (ø)
valohai_cli/utils/commits.py	80.70% <71.42%> (-1.91%)	:arrow_down:

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