Adds two files to gitignore.
`.clang_complete` is created by the clang_complete vim plugin
[https://github.com/Rip-Rip/clang_complete]
`Xdummy.so` is created by compiling the Xdummy module.
This patch splits the main .gitignore with a specific
testcases/.gitignore and also add 4 patterns to the ignore list:
i3-command-parser.stamp
i3-config-parser.stamp
testcases/-
testcases/i3-cfg-for-*