Does not work nicely with custom build script names created by variables

Nobody has claimed this yet.

The completion works fine if your root build script is either named `build.gradle` or its name is set via `rootProject.buildFileName = '...'` or `rootProject.buildFileName = "..."` (actually also `rootProject.buildFileName = "...'` and `rootProject.buildFileName = '..."`, but those will not compile anyway) exactly like that.
You cannot use a variable for the name, you cannot leave out the spaces around the equals sign or add more, it has to be at the start of a line without any preceding whitespaces, and you cannot use other constructs to set the name that are perfectly legal. In my case I do it for all projects like this which of course also does not work:

```
def setBuildFileNameRecursively
setBuildFileNameRecursively = { it ->
it.buildFileName = "${it.name}.gradle"
it.children.each { setBuildFileNameRecursively it }
}
setBuildFileNameRecursively rootProject
```

It would be nice if the completion would not require some narrow range of modification options but simply work. Is it really necessary to determine the root project filename upfront? Can't you use `-p` instead of `-b` for the call and use the project dir as cache key instead of the build file?

No source file or test is named in the issue. Start by finding the bash and zsh completion code that parses rootProject.buildFileName, then compare the proposed -b and -p approaches; done means variable-based and otherwise legal build-file names complete without the listed formatting restrictions.

Written by the indexing model from the issue text.