Read gvfs.* booleans with git when the environment changes config - #2156

Problem and Context

libgit2 does not read the config that GIT_CONFIG_GLOBAL, GIT_CONFIG_SYSTEM, GIT_CONFIG_NOSYSTEM, GIT_CONFIG, GIT_CONFIG_COUNT, or GIT_CONFIG_PARAMETERS select or supply. RetryConfig already detects these variables with LibGit2Repo.EnvironmentChangesGitConfig() and reads its settings with git.exe. The boolean gvfs.* settings did not. They read the user's real config and ignored the value from the environment.

This affects gvfs.trust-pack-indexes, gvfs.prefetch-offload, gvfs.background-cache-auth, gvfs.mount-progress, and gvfs.show-hydration-status. For example, CloneNetworkTests sets gvfs.trust-pack-indexes through GIT_CONFIG_GLOBAL. The clone ignores that value. The test passes only because its value matches the default.

Changes

• When EnvironmentChangesGitConfig() is true, read boolean settings with git config --type=bool. Git normalizes the value, exits with 1 when the key is unset, and fails when the value is not a boolean. A failed read logs a warning and uses the default, as the libgit2 read does.
• Add the LibGit2Repo.TryReadConfigBoolWithGit delegate. The static LibGit2Repo.GetConfigBoolOrDefault now requires it, so a new caller cannot skip the git route.
• Parse the git output in LibGit2Repo. GVFS.Hooks compiles LibGit2Repo.cs without GitProcess, so the hook and the other processes share one parser.
• Add GitProcess.TryGetBoolFromConfig.
• Give LibGit2RepoInvoker an optional git reader. GitRepo supplies one from its enlistment. This covers PrefetchStep, PackfileMaintenanceStep, and GitStatusCache.
• Pass a git reader from CloneVerb, PrefetchVerb, InProcessMount, and GVFS.Hooks. The hook runs git from PATH in the current directory, as its alias check already does. gvfs.mount-progress now uses the static helper instead of its own LibGit2Repo read.
• When the environment does not change config, the reads still use libgit2. The git route adds one git.exe process for each read only in the affected environments.
• Add unit tests that verify the git route, the libgit2 route, an unset key, a failed git read, the output parser, and the git config --type=bool command line.