Autocomplete user-defined pretty formats

[rdnlsmith]

Enumerates any pretty.* keys in the user's Git config and includes
them alongside built-in options like oneline, short, full, etc.
for the --pretty= and --format= parameters of git log and
git show.

The key enumeration is done generically via function gitConfigKeys,
which in theory could be used for enumerating other config values with a
similar structure.

Fixes #786.

[dahlbyk]

Thanks for the contribution! I have a few suggestions - thoughts?

It would be nice to have tests to verify this works as expected with and without custom pretty.* config. For alias setup/teardown, see

posh-git/test/TabExpansion.Tests.ps1

Lines 198 to 207 in d94feed

	BeforeEach {
	[System.Diagnostics.CodeAnalysis.SuppressMessageAttribute('PSUseDeclaredVarsMoreThanAssigments', '')]
	$repoPath = NewGitTempRepo
	# Test with non-standard vsts pr alias name
	&$gitbin config alias.test-vsts-pr "!f() { exec vsts code pr \`"`$`@\`"; }; f"
	}
	AfterEach {
	RemoveGitTempRepo $repoPath
	}

[rdnlsmith]

Updated! I totally agree with the code changes—the way I had it was a case of me trying a little too hard to emulate existing nearby code.

Hopefully the tests I wrote are appropriate!

[dahlbyk]

Nailed it. Nice work, and thanks for the contribution!