Honor .fantomasignore file when processing a folder

[kentcb]

When processing an entire folder with a command such as this:

fantomas --recurse .

the .fantomasignore file is effectively useless because the processSourceString function is generating a fixed file name of either tmp.fs or tmp.fsi. This file name is in turn used by a call to IgnoreFile.isIgnoredFile by formatContentInternalAsync, so a .fantomasignore file like this presents an issue:

*.fs

!/path/to/important.fs

Even when formatting important.fs, it will be "seen" as tmp.fs by formatContentInternalAsync and therefore ignored.

This PR is an intuitive attempt at fixing the issue with minimal change. That said, I do feel like the ignore check is possibly occurring in the wrong place and would be better hoisted to an outer layer of the system. In fact, Program.fs seemingly already does the appropriate ignore checks, so the secondary check within formatContentInternalAsync seems redundant in this context. However, I haven't enough experience with this system to know that one way or another without a lot of digging (open to feedback on that).

[nojaf]

I haven't checked your PR yet, but claiming .fantomasignore is not respected when passing in a folder seems like a bold statement.
I would expect the flow to be:

fantomas/src/Fantomas.CoreGlobalTool/Program.fs

Line 447 in 6869e2e

| InputPath.Folder p1, OutputPath.Notknown -> processFolder p1 p1

fantomas/src/Fantomas.CoreGlobalTool/Program.fs

Line 388 in 6869e2e

let processFolder inputFolder outputFolder =

fantomas/src/Fantomas.CoreGlobalTool/Program.fs

Lines 79 to 91 in 6869e2e

	let rec allFiles isRec path =
	let searchOption =
	(if isRec then
	SearchOption.AllDirectories
	else
	SearchOption.TopDirectoryOnly)
	Directory.GetFiles(path, "*.*", searchOption)
	|> Seq.filter
	(fun f ->
	isFSharpFile f
	&& not (isInExcludedDir f)
	&& not (IgnoreFile.isIgnoredFile f))

So, the check being IgnoreFile.isIgnoredFile f.

The problem you most likely have is probably related to markashleybell/MAB.DotIgnore#9 (comment).

[kentcb]

Please see my PR, which includes tests to prove my bold statement :)

This is the (second) check that fails:

fantomas/src/Fantomas.Extras/FakeHelpers.fs

Line 56 in 6869e2e

if IgnoreFile.isIgnoredFile file then

And it's using the file name determined by:

fantomas/src/Fantomas.CoreGlobalTool/Program.fs

Lines 110 to 114 in 6869e2e

	let fileName =
	if isFsiFile then
	"/tmp.fsi"
	else
	"/tmp.fsx"

[nojaf]

I stand corrected, if the file renaming happens in processSourceString, we cannot do the ignore check in formatContentInternalAsync with the renamed file.
Thank you for this contribution!