ignore: only skip first leading slash

For compatibility with git, only skip the first leading slash in an
ignore file.  That is: `/a.txt` indicates to ignore a file named `a.txt`
at the root.  However `//b.txt` does not indicate that a file named
`b.txt` at the root should be ignored.

src/attr_file.c

@@ -604,7 +604,8 @@ int git_attr_fnmatch__parse(
 		if (*scan == '/') {
 			spec->flags = spec->flags | GIT_ATTR_FNMATCH_FULLPATH;
 			slash_count++;
-			if (pattern == scan)
+
+			if (slash_count == 1 && pattern == scan)
 				pattern++;
 		}
 		/* remember if we see an unescaped wildcard in pattern */