Add README and update flow for GitHub demo data (#129)

Keeping the GitHub demo data up-to-date is important, and there isn't
good documentation for how to do that.

This commit adds a short README.md for the demo data, and adds an update
flag to fetchGithubRepoTest.sh that can be used to easily update it.

Test plan:
Modify example-repo.json (e.g. by deleting it entirely). Run
fetchGithubRepoTest.sh -u and confirm that the data was regenerated
without change. Run fetchGithubRepoTest.sh and confirm the test passes.

Note: The end cursor is sensitive to the timezone, which seems to be
cached with the GitHub token. An erroneous switch to Israel timezone
made it into master; this commit reverts back to US/Pacific.

src/plugins/github/demoData/README.md

@@ -0,0 +1,22 @@
+# DemoData Instructions
+
+This folder contains example data for testing the GitHub plugin. The example
+data is generated by running our GitHub API fetch against the SourceCred
+example repo at https://github.com/sourcecred/example-repo/.
+
+The demo data should be regenerated any time either the example repository or
+the GitHub api query changes. To regenerate the demo data, run the following
+command:
+
+```shell
+$ GITHUB_TOKEN="your_token_here" src/plugins/github/fetchGithubRepoTest.sh -u
+```
+
+You can also just verify that the data is up-to-date by running:
+```shell
+$ GITHUB_TOKEN="your_token_here" src/plugins/github/fetchGithubRepoTest.sh
+```
+
+There is a known issue where GitHub's end cursor output depends on your current
+timezone. The example data is canonically generated from the US/Pacific
+timezone.

src/plugins/github/demoData/example-repo.json

@@ -295,7 +295,7 @@
                                 }
                             ],
                             "pageInfo": {
-                                "endCursor": "Y3Vyc29yOnYyOpO5MjAxOC0wMy0wMVQwNjoyNDo1NiswMjowMLkyMDE4LTAzLTAxVDA2OjI0OjU2KzAyOjAwzgX6q7Y=",
+                                "endCursor": "Y3Vyc29yOnYyOpO5MjAxOC0wMi0yOFQyMDoyNDo1Ni0wODowMLkyMDE4LTAyLTI4VDIwOjI0OjU2LTA4OjAwzgX6q7Y=",
                                 "hasNextPage": false
                             }
                         },

src/plugins/github/fetchGithubRepoTest.sh

@@ -2,27 +2,58 @@
 
 set -eu
 
-main() {
-  if ! [ -d bin ]; then
-    printf >&2 'Backend applications have not been built.\n'
-    printf >&2 'Please run "yarn backend".\n'
-    return 1
-  fi
+data_file=src/plugins/github/demoData/example-repo.json
+
+usage() {
+  printf 'usage: %s [-u|--updateSnapshot] [--help]\n' "$0"
+  printf 'Required environment variables:\n'
+  printf '  GITHUB_TOKEN: A 40-character hex string API token.\n'
+  printf 'Flags:\n'
+  printf '  -u|--updateSnapshot\n'
+  printf '      Update the stored file instead of checking its contents\n'
+  printf '  --help\n'
+  printf '      Show this message\n'
+}
+
+fetch() {
   if [ -z "${GITHUB_TOKEN:-}" ]; then
     printf >&2 'Please set the GITHUB_TOKEN environment variable\n'
     printf >&2 'to a 40-character hex string API token from GitHub.\n'
     return 1
   fi
-  output="$(mktemp)"
+  yarn backend >&2
   node bin/fetchAndPrintGithubRepo.js \
-    sourcecred example-repo "${GITHUB_TOKEN}" \
-    >"${output}" \
-    ;
-  diff -uw \
-    src/plugins/github/demoData/example-repo.json \
-    "${output}" \
-    ;
+    sourcecred example-repo "${GITHUB_TOKEN}"
+}
+
+check() {
+  output="$(mktemp)"
+  fetch >"${output}"
+  diff -uw "${data_file}" "${output}"
   rm "${output}"
 }
 
+update() {
+  fetch >"${data_file}"
+}
+
+main() {
+  cd "$(git rev-parse --show-toplevel)"
+  if [ $# -eq 0 ]; then
+    check
+  elif [ $# -eq 1 ]; then
+    if [ "$1" = "-u" ] || [ "$1" = "--updateSnapshot" ]; then
+      update
+    elif [ "$1" = "--help" ]; then
+      usage
+    else
+      usage >&2
+      return 1
+    fi
+  else
+    usage >&2
+    return 1
+  fi
+}
+
 main "$@"