checkout-branch example

[dlambda]

This PR adds a checkout-branch example and some minor fixes for testing.

It took me a while to figure out how to check-out a branch in my cloned repo, and thought this might save some time and could be useful helping someone else to do so.
Originally, after some digging, I used the method used in this issue: src-d/go-git#777
However, realised it was copying all references, and causes faulty HEAD references in some cases (git "refname HEAD is ambiguous" warning I think, but can't remember exactly now). Couldn't find any examples of fetching a single branch, hence thought this might be useful.

[AdamRussak]

im not sure why it is not merged.
but it assisted me to understand how to do a branch checkout via code.

[nustiueudinastea]

This example also helped me. So if possible, we should merge it. Thanks!

[jmcristobal]

Please, merge this example. It's super useful.

[pjbgf]

@dlambda do you mind rebasing this please?

[dlambda]

@pjbgf Done thanks. It's been a while and needed a bit of re-familiarisation for the example tests.
I included some of the newer tests and excluded some borked ones.

go test -v _examples/common_test.go _examples/common.go --examples
...
--- PASS: TestExamples (19.21s)
    --- PASS: TestExamples/blame (1.27s)
    --- PASS: TestExamples/branch (1.14s)
    --- PASS: TestExamples/checkout (1.58s)
    --- PASS: TestExamples/checkout-branch (1.42s)
    --- PASS: TestExamples/clone (1.54s)
    --- PASS: TestExamples/commit (0.49s)
    --- PASS: TestExamples/context (1.10s)
    --- PASS: TestExamples/custom_http (1.07s)
    --- PASS: TestExamples/find-if-any-tag-point-head (0.47s)
    --- PASS: TestExamples/log (1.15s)
    --- PASS: TestExamples/ls-remote (0.81s)
    --- PASS: TestExamples/merge_base (0.48s)
    --- PASS: TestExamples/open (0.48s)
    --- PASS: TestExamples/progress (1.14s)
    --- PASS: TestExamples/pull (1.12s)
    --- PASS: TestExamples/push (0.60s)
    --- PASS: TestExamples/remotes (1.11s)
    --- PASS: TestExamples/revision (0.48s)
    --- PASS: TestExamples/showcase (1.14s)
    --- PASS: TestExamples/tag (0.49s)
PASS
ok  	command-line-arguments	29.750s

It would be great if this were to be auto-checked when people add new examples.

[pjbgf]

It would be great if this were to be auto-checked when people add new examples.

@dlambda completely agree. Please add $(GOTEST) -v _examples/common_test.go _examples/common.go --examples after https://github.com/go-git/go-git/blob/master/Makefile#L30C2-L30C11.

Please reword the commit in line with the guidelines and squash the commits.

[pjbgf]

Fixes #912

[dlambda]

@pjbgf - thanks: done. Apologies for the delayed response - was a busy time.

[pjbgf]

@dlambda thanks for working on this. 🙇