fix!: branch setup for git init

[jbrockopp]

xref: go-vela/community#816

Based off of #122

This change ensures we set the expected branch when invoking git init.

• for pull_request events, this sets the branch for git init from VELA_PULL_REQUEST_SOURCE
• for all other events, this sets the branch for git init from VELA_BUILD_BRANCH

To test this, I set up a repo with a default branch of main and a .vela.yml that looks like:

version: "1"

metadata:
  clone: false

steps:
  - name: clone
    image: vela-git:local # built this image with 'make build-static; make docker-build'

  - name: context
    image: alpine:latest
    commands:
      - apk add git
      - git branch
      - git status
      - printenv | sort

Then, I created a new context branch which I modified the README.md and opened a PR that targets main.

Before this change, for both the pull_request and push events, here's a snippet of output from the context step:

<other output>
$ git branch
* main
$ git status
On branch main
nothing to commit, working tree clean

After this change, here's a snippet of output from the context step:

<other output>
$ git branch
* context
$ git status
On branch context
nothing to commit, working tree clean

[codecov]

Codecov Report

Merging #138 (6d793ed) into main (b41373c) will decrease coverage by 1.53%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##             main     #138      +/-   ##
==========================================
- Coverage   49.05%   47.52%   -1.53%     
==========================================
  Files           6        6              
  Lines         424      425       +1     
==========================================
- Hits          208      202       -6     
- Misses        205      210       +5     
- Partials       11       13       +2     

Impacted Files	Coverage Δ
cmd/vela-git/main.go	0.00% <0.00%> (ø)
cmd/vela-git/repo.go	100.00% <ø> (ø)
cmd/vela-git/build.go	100.00% <100.00%> (ø)
cmd/vela-git/plugin.go	44.28% <100.00%> (-8.58%)	⬇️

[jbrockopp]

We specify the VELA_PULL_REQUEST_SOURCE environment variable first intentionally.

This environment variable is ONLY set for pull_request events.

This ensures the CLI library we use will try to set this value from that variable first.

In the event this plugin runs for a non pull_request event, we then source from VELA_BUILD_BRANCH.

If all else fails, we have a fallback value of master to mirror the build.ref flag:

vela-git/cmd/vela-git/main.go

Lines 82 to 88 in b41373c

	&cli.StringFlag{
	EnvVars: []string{"PARAMETER_REF", "GIT_REF", "VELA_BUILD_REF"},
	FilePath: "/vela/parameters/git/ref,/vela/secrets/git/ref",
	Name: "build.ref",
	Usage: "commit reference to clone from the repo",
	Value: "refs/heads/master",
	},

[plyr4]

love it, thanks @jbrockopp
agreed its a fix, should we mark it breaking?

[jbrockopp]

love it, thanks @jbrockopp agreed its a fix, should we mark it breaking?

I'm not entirely sure the procedure for bug fixes that are breaking changes 😅

At any rate, I updated the PR description with a ! to denote that; rather be safe than sorry 👍