Multiple file sources

[spenserblack]

Right now, the file source is a git repository via git2. But the analyzer is pretty flexible, and basically just needs to receive a filepath and bytes. So it might be cool to have a trait like FileSource, which would require an iterator method that returns a path and bytes and a static open method.

Besides a git repository, another source could be the filesystem.

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This could also perhaps lead to multiple binaries -- git-gengo would analyze a git repository, and gengo would analyze a directory.

[spenserblack]

#150 (comment) Different file sources may have utilities that are too specific for a generic FileSource trait. So it might make more sense to have multiple implementations for multiple file sources.

[Byron]

I also recommend to move the git-specific code into its own submodule to pave the way to one day allow it (and its dependencies) to be turned off. gengo can compile much faster once gix isn't in the dependency tree anymore.

[spenserblack]

Notes based on discussion in #174:

• Allowing sources to define their own overrides with MySource::with_overrides(overrides) could be nice, especially for more generic implementations
  • based on this, some sort of Overrides::from_gitattributes() could be nice