mingw: allow absolute paths without drive prefix

When specifying an absolute path without a drive prefix, we convert that
path internally. Let's make sure that we handle that case properly, too
;-)

This fixes the command

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

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

compat/mingw.c

@@ -1071,11 +1071,19 @@ unsigned int sleep (unsigned int seconds)
 char *mingw_mktemp(char *template)
 {
 	wchar_t wtemplate[MAX_PATH];
+	int offset = 0;
+
 	if (xutftowcs_path(wtemplate, template) < 0)
 		return NULL;
+
+	if (is_dir_sep(template[0]) && !is_dir_sep(template[1]) &&
+	    iswalpha(wtemplate[0]) && wtemplate[1] == L':') {
+		/* We have an absolute path missing the drive prefix */
+		offset = 2;
+	}
 	if (!_wmktemp(wtemplate))
 		return NULL;
-	if (xwcstoutf(template, wtemplate, strlen(template) + 1) < 0)
+	if (xwcstoutf(template, wtemplate + offset, strlen(template) + 1) < 0)
 		return NULL;
 	return template;
 }

t/t5580-unc-paths.sh

@@ -33,7 +33,7 @@ case "$UNCPATH" in
 	;;
 esac
 
-test_expect_failure 'clone into absolute path lacking a drive prefix' '
+test_expect_success 'clone into absolute path lacking a drive prefix' '
 	USINGBACKSLASHES="$(echo "$WITHOUTDRIVE"/without-drive-prefix |
 		tr / \\\\)" &&
 	git clone . "$USINGBACKSLASHES" &&