gist-logs: grab files in subdirectories too

[gromgit]

#11594 introduces subdirectories for additional logs (e.g. CMake build logs), so brew gist-logs can now throw errors like:

$ brew gist-logs gcc
Error: Is a directory @ io_fread - /home/yz/.cache/Homebrew/Logs/gcc/build

This PR grabs log files recursively, but GitHub's gists can't deal with directory separators in relative paths. Therefore, I'm also replacing all / with :, since I assume Windows support is not on the cards any time soon. This means gists will now contain paths like build:amalgamation:CMakeFiles:CMakeOutput.log, which I find more-or-less readable, but I'm open to suggestions for directory separator replacements. (I didn't use _ because that's already a common character in filenames.)

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[BrewTestBot]

Review period will end on 2021-08-03 at 06:10:15 UTC.

[MikeMcQuaid]

Nice work!

[BrewTestBot]

Review period ended.