gvfs: allow overriding core.gvfs

In earlier versions of `microsoft/git`, we found a user who had set
`core.gvfs = false` in their global config. This should not have been
necessary, but it also should not have caused a problem. However, it
did.

The reason was that `gvfs_load_config_value()` was called from
`config.c` when reading config key/value pairs from all the config
files. The local config should override the global config, and this is
done by `config.c` reading the global config first then reading the
local config. However, our logic only allowed writing the `core_gvfs`
variable once.

In v2.51.0, we had to adapt to upstream changes that changed way the
`core.gvfs` config value is read, and the special handling is no longer
necessary, yet we still want the test case that ensures that this bug
does not experience a regression.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>

Author: derrickstolee

t/t0021-conversion.sh

@@ -348,6 +348,10 @@ test_expect_success "filter: smudge filters blocked when under GVFS" '
 	test_config filter.empty-in-repo.smudge "echo smudged && cat" &&
 	test_config core.gvfs 64 &&
 
+	test_must_fail git checkout &&
+
+	# ensure the local core.gvfs setting overwrites the global setting
+	git config --global core.gvfs false &&
 	test_must_fail git checkout
 '