Fix path/branch extraction removing every /blob/ and /tree/ marker, not just the leading one (fixes #150)

[patchwright]

Fixes #150.

Re-opened from #149 with the branch renamed to bugfix/issue-150-fix-blob-tree-prefix-strip so the towncrier branch-name check passes (per @protoroto's request). Changelog fragment renamed to changes/150.bugfix. Same commit and diff as #149 — no code changes.

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Description

path and branch are extracted from the matched URL by removing the leading
/blob/, /tree/, /-/blob/ or /-/tree/ marker. The code did this with
str.replace(marker, ""), which removes every occurrence of the marker, not
just the leading one. When the file path or branch name legitimately contains the
same segment again, the result is silently corrupted.

For example:

>>> import giturlparse
>>> giturlparse.parse("https://github.com/owner/repo/blob/main/src/blob/utils.py").path
'main/srcutils.py'        # expected: 'main/src/blob/utils.py'
>>> giturlparse.parse("https://github.com/owner/repo/tree/feature/tree/x").branch
'featurex'                # expected: 'feature/tree/x'

The leading marker is already guaranteed by the preceding startswith(...) check,
so the fix is to slice off exactly that prefix instead of calling replace:

data["path"] = data["path_raw"][len("/blob/"):]

Slicing is used (rather than str.removeprefix) to remain compatible with the
declared python_requires = >=3.8.

Fixed in both the GitHub and GitLab platforms.

References

No existing issue.

Checklist

• Code lint checked via inv lint (ruff, black, isort all clean on the changed files)
• Tests added (regression cases for nested /blob/ paths and /tree/ branch names on both GitHub and GitLab; verified to fail on the previous code and pass with the fix)
• Changelog fragment added under changes/

[protoroto]

@patchwright Towncrier kept complaining (somehow he was still trying with the old branch name)... anyway, I've merged and released 0.15.0 version :) thanks again for contributing!