Inherited GIT_ICASE/GLOB/NOGLOB_PATHSPECS breaks every path-taking verb, and inherited GIT_QUARANTINE_PATH breaks every ref-updating verb: the environment overlay doesn't clear them #237

What's wrong

RunCommandGitProcessRunner.EnvironmentOverlay (GitIntegration/Execution/RunCommandGitProcessRunner.cs:61-75) clears inherited variables that would override the library's own arguments, such as GIT_DIR, GIT_INDEX_FILE and GIT_DIFF_OPTS (#139). Two more groups of variables still get through, and each one breaks a whole class of verbs.

1. The global pathspec variables. GitCommandBuilder.BuildArguments adds --literal-pathspecs whenever a builder passes caller paths (GitIntegration/Builders/GitCommandBuilder.cs:154-157). That covers Add().ForPath, Diff/Patch/Log/RevList().ForPath, and always Unstage. Git will not combine the literal setting with any other global pathspec setting:

$ GIT_ICASE_PATHSPECS=1 git --literal-pathspecs add -- a.txt
fatal: global 'literal' pathspec setting is incompatible with all other global pathspec settings   (exit 128)

GIT_GLOB_PATHSPECS=1 and GIT_NOGLOB_PATHSPECS=1 fail the same way. Some users set GIT_ICASE_PATHSPECS in their shell profile on case-insensitive filesystems. For them, every path-filtered verb fails.

2. GIT_QUARANTINE_PATH. Git sets this for pre-receive and update hooks, and while it is set git refuses every ref update. A tool built on this library that runs from such a hook cannot commit, branch, tag, fetch or push:

fatal: ref updates forbidden inside quarantine environment

The overlay already clears GIT_OBJECT_DIRECTORY and GIT_ALTERNATE_OBJECT_DIRECTORIES, which are the other half of the quarantine environment.

Reproduced through the library (git 2.43, net10.0, HEAD 65984d7)

• With GIT_ICASE_PATHSPECS=1 set, repo.Add().ForPath("b.txt").TryExecuteAsync() returns Success=False with the "incompatible with all other global pathspec settings" error.
• With GIT_QUARANTINE_PATH=/tmp/x set and a file staged, repo.Commit("msg").TryExecuteAsync() returns Success=False with "ref updates forbidden inside quarantine environment".

Suggested fix / acceptance criteria

• Add GIT_ICASE_PATHSPECS, GIT_GLOB_PATHSPECS, GIT_NOGLOB_PATHSPECS and GIT_QUARANTINE_PATH to EnvironmentOverlay with the value null.
  • Optionally add GIT_LITERAL_PATHSPECS as well, so builders that do not opt in behave the same whatever the parent set.
• Extend the overlay's <remarks> to cover both groups.
• Add tests to GitIntegration.Test/Integration/GitInheritedEnvironmentTests.cs using EnvironmentScope:
  • With GIT_ICASE_PATHSPECS=1, Add().ForPath(...) succeeds and stages exactly that file.
  • With GIT_QUARANTINE_PATH set, Commit(...) succeeds.