fix: avoid unnecessary git checks

[i4ki]

Now the git checks only happen in run and with --changed flag.

[codecov]

Codecov Report

Merging #339 (cd5b435) into main (6591957) will decrease coverage by 0.49%.
The diff coverage is 30.18%.

@@            Coverage Diff             @@
##             main     #339      +/-   ##
==========================================
- Coverage   68.23%   67.73%   -0.50%     
==========================================
  Files          36       36              
  Lines        6670     6670              
==========================================
- Hits         4551     4518      -33     
- Misses       1859     1902      +43     
+ Partials      260      250      -10     

Flag	Coverage Δ
tests	67.73% <30.18%> (-0.50%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cmd/terramate/cli/cli.go	5.81% <2.32%> (-0.05%)	⬇️
cmd/terramate/cli/project.go	44.18% <46.29%> (-17.52%)	⬇️
git/git.go	74.16% <50.00%> (-0.25%)	⬇️
test/sandbox/git.go	74.82% <100.00%> (+0.53%)	⬆️
manager.go	63.10% <0.00%> (+0.24%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6591957...cd5b435. Read the comment docs.

[katcipis]

LGTM. Thanks for improving the changes and the tests 💯