Fix and speed up the file list for the "Enter path to filter by" feature

[stefanhaller]

The file suggestions list that appears when you choose "Enter path to filter by" from the Filtering menu was previously implemented by walking the file system to collect all files, and then filter out the ones that are git-ignored. This approach had several problems:

• for certain .gitignore configurations there was a bug in the code that filters out ignored files, causing the list to be empty. See File suggestion not working #2642 (comment) for details.
• the list included files from submodules. While at first glance this might sound useful, it doesn't work: choosing a file from the list shows an empty log. Lazygit would have to switch to the submodule first to show the log, which would be unexpected in the context of the Filtering dialog, so it's better to not show these files, and require the user to enter a submodule explicitly before they can filter on a file in it.
• it was rather slow.

This PR improves all these by using a call to git ls-files to list the repositories files.

Here are some rough hand-timed measurements (from opening the Filtering dialog until the file list shows) for various repos:

	before	after
lazygit (3'200 files)	~1s	not measurable
my work repo (90'000 files)	7s	<1s
llvm (150'000 files)	5s	~1s

Fixes #2642

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 9d249631	✅ 91.67%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (9d24963)	Report Missing	Report Missing	Report Missing
Head commit (b06c2bb)	59132	51420	86.96%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#5056)	24	22	91.67%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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Footnotes

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