[BUG] exo manifest format panics if read file is changed

[BenElgar]

❯ exo manifest format exo.hcl > exo.hcl
vpanic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x8c0e3c]

goroutine 1 [running]:
github.com/deref/exo/internal/manifest/exohcl.(*FormatVersion).Analyze(0xc00000f6f8, 0xc000414720)
        /home/ben/Programming/exo/internal/manifest/exohcl/version.go:37 +0x3c
github.com/deref/exo/internal/manifest/exohcl.(*Manifest).Analyze(0xc0004103c0, 0xc000414720)
        /home/ben/Programming/exo/internal/manifest/exohcl/manifest.go:41 +0x285
github.com/deref/exo/internal/manifest.(*Loader).Load(0xc0002efd10, 0x0)
        /home/ben/Programming/exo/internal/manifest/manifest.go:75 +0x295
github.com/deref/exo/internal/cli.loadManifest({0xf0bcc8, 0xc0004146f0}, {0xef74e0, 0xc000010020}, {0x7ffed199c32e, 0x7})
        /home/ben/Programming/exo/internal/cli/manifest.go:51 +0x1ba
github.com/deref/exo/internal/cli.glob..func25(0x140ed00, {0xc0003b3e00, 0x1, 0x1})
        /home/ben/Programming/exo/internal/cli/manifest_format.go:20 +0x50
github.com/spf13/cobra.(*Command).execute(0x140ed00, {0xc0003b3dd0, 0x1, 0x1})
        /home/ben/go/pkg/mod/github.com/spf13/cobra@v1.2.1/command.go:856 +0x60e
github.com/spf13/cobra.(*Command).ExecuteC(0x1410b00)
        /home/ben/go/pkg/mod/github.com/spf13/cobra@v1.2.1/command.go:974 +0x3bc
github.com/spf13/cobra.(*Command).Execute(...)
        /home/ben/go/pkg/mod/github.com/spf13/cobra@v1.2.1/command.go:902
github.com/deref/exo/internal/cli.Main()
        /home/ben/Programming/exo/internal/cli/root.go:51 +0x6a
main.main()
        /home/ben/Programming/exo/main.go:6 +0x17

[brandonbloom]

I wouldn't expect this usage to work because the shell truncates (deletes & recreates?) the destination file.

However, it definitely shouldn't crash! 41b229d fixes that.

Though I'll take this as a feature request for being able to do a format-in-place on the manifest file & will follow-up with that soon.