add support for gist.github.com

[gnawhleinad]

I left a todo for (future?) GitHub Enterprise Server support because gists could be either on gist. or /gist depending on subdomain isolation.

[gnawhleinad]

Looks like gist.github.com doesn't like it if you set Authorization headers on a fetch...

$ git remote add origin https://gist.github.com/d530295e01f1203e063a671b106f6be3
$ git config --local "http.https://gist.github.com/.extraheader" "AUTHORIZATION: basic x-access-token:hunter2"
$ git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --depth=1 origin +refs/heads/master:refs/remotes/origin/master
fatal: unable to access 'https://gist.github.com/d530295e01f1203e063a671b106f6be3/': The requested URL returned error: 400
$ git config --local --unset "http.https://gist.github.com/.extraheader"
$ git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --depth=1 origin +refs/heads/master:refs/remotes/origin/master
remote: Enumerating objects: 3, done.
remote: Counting objects: 100% (3/3), done.
remote: Total 3 (delta 0), reused 0 (delta 0), pack-reused 0
Unpacking objects: 100% (3/3), 232 bytes | 232.00 KiB/s, done.
From https://gist.github.com/d530295e01f1203e063a671b106f6be3
 * [new branch]      master     -> origin/master

[gnawhleinad]

Looks like gist.github.com doesn't like it if you set Authorization headers on a fetch...

NVM, it just doesn't like incorrect credentials. I was trying invalid credentials on a public gist and thought it failed because credentials were present...

[gnawhleinad]

added this back from 00a3be8

let me know if you'd like me to change src/git-source-provider.ts instead.