Handle tags that do not follow semantic versioning

[pete0emerson]

Your documentation indeed points out that tags need to follow SemVer:

Note that fetch assumes git tags are specified according to Semantic Versioning principles.

However, git tags that are not semantically versioned are valid tags, and if one exists in the source repository, the tool breaks, even if you are not requesting a --tag.

I suggest that tags should not be required to be semantically versioned unless a Tag Contraint Expression is specified.

This would allow the tool to work in repositories where non-SemVar tags exist.

To replicate:

fetch $ git rev-parse HEAD
83ed55048791ec5385d4143a78798e2a6c82b9f8

fetch $ ./fetch --repo=https://github.com/pete0emerson/demo --branch=main demo
Downloading latest commit from branch "main" of https://github.com/pete0emerson/demo ...
Extracting files from <repo>/ to demo ...
Download and file extraction complete.

demo $ git tag banana
demo $ git push --tags
Total 0 (delta 0), reused 0 (delta 0)
To https://github.com/pete0emerson/demo.git
 * [new tag]         banana -> banana

fetch $ ./fetch --repo=https://github.com/pete0emerson/demo --branch=main demo
ERROR: Error occurred while computing latest tag that satisfies version contraint expression: -1 - Malformed version: banana