mingw: prevent regressions with "drive-less" absolute paths

On Windows, there are several categories of absolute paths. One such
category starts with a backslash and is implicitly relative to the drive
associated with the current working directory. Example:

	c:
	git clone https://github.com/git-for-windows/git \G4W

should clone into C:\G4W.

Back in 2017, Juan Carlos Arevalo Baeza reported a bug in Git's handling
of those absolute paths was identified, and fixed. Let's make sure that
it stays fixed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/t5580-unc-paths.sh

@@ -20,14 +20,11 @@ fi
 UNCPATH="$(winpwd)"
 case "$UNCPATH" in
 [A-Z]:*)
+	WITHOUTDRIVE="${UNCPATH#?:}"
 	# Use administrative share e.g. \\localhost\C$\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 "$UNCPATH" || {
-		skip_all='could not access administrative share; skipping'
-		test_done
-	}
+	UNCPATH="//localhost/${UNCPATH%%:*}\$$WITHOUTDRIVE"
 	;;
 *)
 	skip_all='skipping UNC path tests, cannot determine current path as UNC'
@@ -35,6 +32,18 @@ case "$UNCPATH" in
 	;;
 esac
 
+test_expect_success 'clone into absolute path lacking a drive prefix' '
+	USINGBACKSLASHES="$(echo "$WITHOUTDRIVE"/without-drive-prefix |
+		tr / \\\\)" &&
+	git clone . "$USINGBACKSLASHES" &&
+	test -f without-drive-prefix/.git/HEAD
+'
+
+test -d "$UNCPATH" || {
+	skip_all='could not access administrative share; skipping'
+	test_done
+}
+
 test_expect_success setup '
 	test_commit initial
 '