mingw: demonstrate a regression pushing to UNC paths

On Windows, there are "UNC paths" to access network (AKA shared) folders,
of the form \\server\sharename\directory. This provides a convenient way
for Windows developers to share their Git repositories without having to
have a dedicated server.

Git for Windows v2.11.0 introduced a regression where pushing to said UNC
paths no longer works, although fetching and cloning still does.

Demonstrate what is the problem, using so-called "administrative shares":
disk volumes are automatically shared under certain circumstances, e.g.
the C: drive is shared as \\localhost\c$. The test needs to be skipped if
the current directory is inaccessible via said administrative share, of
course.

Original-report: #979
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/t5580-clone-push-unc.sh

@@ -0,0 +1,46 @@
+#!/bin/sh
+
+test_description='various UNC path tests (Windows-only)'
+. ./test-lib.sh
+
+if ! test_have_prereq MINGW; then
+	skip_all='skipping UNC path tests, requires Windows'
+	test_done
+fi
+
+UNCPATH="$(pwd)"
+case "$UNCPATH" in
+[A-Z]:*)
+	# Use administrative share e.g. \\localhost\C$\git-sdk-64\usr\src\git
+	# (we use forward slashes here because MSYS2 and Git accept them, and
+	# they are easier on the eyes)
+	UNCPATH="//localhost/${UNCPATH%%:*}\$/${UNCPATH#?:}"
+	test -d "$UNCPATH" || {
+		skip_all='could not access administrative share; skipping'
+		test_done
+	}
+	;;
+*)
+	skip_all='skipping UNC path tests, cannot determine current path as UNC'
+	test_done
+	;;
+esac
+
+test_expect_success setup '
+	test_commit initial
+'
+
+test_expect_success clone '
+	git clone "file://$UNCPATH" clone
+'
+
+test_expect_failure push '
+	(
+		cd clone &&
+		git checkout -b to-push &&
+		test_commit to-push &&
+		git push origin HEAD
+	)
+'
+
+test_done