t1508: Be more clever than msys path substitution

A string of the form "@/abcd" is considered a file path
by the msys layer and therefore translated to a windows path.

Here the trick is to double the slashes.

The msys patch translation can be studied with the following
test program:

 #include <stdio.h>
 #include <stdlib.h>

 int main(int argc, char** argv)
 {
 unsigned int i;
 for(i=1; i < argc; i++)
 	printf("argv[%d]=%s\n",i, argv[i]);

 exit(0);
 }

Signed-off-by: Thomas Braun <[email protected]>

Author: t-b

t/t1508-at-combinations.sh

@@ -35,7 +35,7 @@ test_expect_success 'setup' '
 	git checkout -b upstream-branch &&
 	test_commit upstream-one &&
 	test_commit upstream-two &&
-	git checkout -b @/at-test &&
+	git checkout -b @//at-test &&
 	git checkout -b @@/at-test &&
 	git checkout -b @at-test &&
 	git checkout -b old-branch &&
@@ -64,7 +64,7 @@ check "@{-1}@{u}@{1}" commit master-one
 check "@" commit new-two
 check "@@{u}" ref refs/heads/upstream-branch
 check "@@/at-test" ref refs/heads/@@/at-test
-check "@/at-test" ref refs/heads/@/at-test
+check "@//at-test" ref refs/heads/@/at-test
 check "@at-test" ref refs/heads/@at-test
 nonsense "@{u}@{-1}"
 nonsense "@{0}@{0}"