Introduce the function strip_path_suffix()

The function strip_path_suffix() will try to strip a given suffix from
a given path.  The suffix must start at a directory boundary (i.e. "core"
is not a path suffix of "libexec/git-core", but "git-core" is).

Arbitrary runs of directory separators ("slashes") are assumed identical.

Example:

	prefix = strip_path_suffix("C:\\msysgit/\\libexec\\git-core",
			"libexec///git-core");

will set prefix to "C:\\msysgit".

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

test-path-utils.c

@@ -22,5 +22,10 @@ int main(int argc, char **argv)
 		printf("%d\n", len);
 	}
 
+	if (argc == 4 && !strcmp(argv[1], "strip_path_suffix")) {
+		char *prefix = strip_path_suffix(argv[2], argv[3]);
+		printf("%s\n", prefix ? prefix : "(null)");
+	}
+
 	return 0;
 }