Simplify .gitignore: projects/* => projects/*/

Avoid listing inclusions (like `!projects/LLVMBuild.txt`) for files
directly underneath `projects/` in `.gitignore`.  Instead, change the
`projects/*` exclusion to the more specific `projects/*/`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240973 91177308-0d34-0410-b5e6-96231b3b80d8

.gitignore

@@ -43,10 +43,7 @@ autoconf/autom4te.cache
 # Directories to ignore (do not add trailing '/'s, they skip symlinks).
 #==============================================================================#
 # External projects that are tracked independently.
-projects/*
-!projects/CMakeLists.txt
-!projects/LLVMBuild.txt
-!projects/Makefile
+projects/*/
 # Clang, which is tracked independently.
 tools/clang
 # LLDB, which is tracked independently.