feat: Allow setting custom Git commit messages

[jannfis]

This PR introduces the possibility to set custom Git commit messages using simple templates.

Closes #169

[codecov]

Codecov Report

Merging #193 (190d600) into master (0257771) will decrease coverage by 0.34%.
The diff coverage is 61.60%.

@@            Coverage Diff             @@
##           master     #193      +/-   ##
==========================================
- Coverage   67.12%   66.77%   -0.35%     
==========================================
  Files          19       20       +1     
  Lines        1466     1502      +36     
==========================================
+ Hits          984     1003      +19     
- Misses        391      406      +15     
- Partials       91       93       +2     

Impacted Files	Coverage Δ
pkg/argocd/git.go	59.80% <59.80%> (ø)
pkg/argocd/update.go	70.23% <80.00%> (+1.54%)	⬆️

---

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 e5ced9d...190d600. Read the comment docs.