Read git config strings via a config snapshot in LibGit2Repo

[tyrielv]

Problem

LibGit2Repo.GetConfigString read a git config value with the native
git_config_get_string. It called that function on the live config object
returned by git_repository_config. libgit2 does not allow
git_config_get_string on a live config and fails with get_string called on a live config object. The read then threw LibGit2Exception, so callers
silently fell back to their default value instead of the configured value.

A second, latent defect sat in the same P/Invoke: git_config_get_string
returns a borrowed const char* owned by the config, but it was marshalled
as an out string. The interop marshaller frees an out-string buffer with
CoTaskMemFree — a mismatched-allocator free of memory libgit2 still owns,
which corrupts the heap. The live-config error masked this because the call
never returned a string; fixing the first bug makes the call succeed and would
expose the corruption.

Fix

• Take a git_config_snapshot of the live config and read the string from the
  snapshot, then free the snapshot. git_config_get_string is valid on a
  snapshot config.
• Change the git_config_get_string binding to return an IntPtr and copy the
  value with Marshal.PtrToStringUTF8, which never frees the borrowed pointer.
  This matches the manual marshalling already used by GitConfigEntry.
• A missing key still returns null, so the caller default applies without a
  spurious error. git_config_get_bool parses its value rather than returning
  a borrowed pointer, so GetConfigBool is unchanged.

Tests

Adds LibGit2ConfigTests, a functional test that exercises the real libgit2
(git2.dll) config path against a plain on-disk git repository: it sets a
string and a bool config value, reads them back through LibGit2Repo, and
asserts a missing key returns null. A unit test cannot cover this because the
unit tests mock the native layer; the failure only appears through the real
P/Invoke. The test fails before this change and passes after.

Unit suite: 909 passed, 0 failed locally. The functional test was not run
locally (the native C++ projects need a toolset not available on this machine);
the PR validation build runs it.