langserver: Avoid re-processing open files after file saving

[radeksimko]

Closes #1366

Context

A common use case for workspace/didChangeWatchedFiles is to receive updates about files changed outside of the IDE - such as when the user runs git checkout to switch branches.

The LSP spec isn't 100% clear about whether clients should always send workspace/didChangeWatchedFiles also for open files. A popular language client (VS Code) always does - i.e. we receive both textDocument/didSave on save and then subsequently workspace/didChangeWatchedFiles even if the file content has not changed - only the modification timestamp has.

This leads us to processing the exact same files (content-wise) multiple times in row (1st on textDocument/didChange 2nd and any other time on workspace/didChangeWatchedFiles as followed after textDocument/didSave), in turn wasting resources.

Solution

Reduce the work we do after file saving by assuming that changes have already been processed as part of textDocument/didChange.

[dbanck]

👍

[github-actions]

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