Add vi swap files to .gitignore

BUG=none
R=dcarney@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/181263002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19559 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

.gitignore

@@ -22,6 +22,7 @@
 *~
 .cpplint-cache
 .d8_history
+.*.sw?
 bsuite
 d8
 d8_g