MessageWithDiff panics when the message is too long

[mvladev]

When running the following test

It("handles negative padding length", func() {
	stringWithB := "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
	stringWithZ := "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaazaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
	longMessage := "to equal very long message"

	Expect(MessageWithDiff(stringWithB, longMessage, stringWithZ)).Should(Equal(expectedDiffLongMessage))
})

It panics with:

•! Panic [0.000 seconds]
Format
/gopath/github.com/gomega/format/format_test.go:95
  MessageWithDiff
  /gopath/github.com/gomega/format/format_test.go:133
    handles negative padding length [It]
    /gopath/github.com/gomega/format/format_test.go:177

    Test Panicked
    strings: negative Repeat count
    /usr/local/opt/go/libexec/src/strings/strings.go:529

    Full Stack Trace
    strings.Repeat(0x13bd790, 0x1, 0xfffffffffffffffd, 0x13, 0x9)
        /usr/local/opt/go/libexec/src/strings/strings.go:529 +0x5e5
    github.com/onsi/gomega/format.MessageWithDiff(0x13d9f7d, 0x65, 0x13cbbf7, 0x1a, 0x13d9eb3, 0x65, 0xc00039e001, 0x3f)
        /gopath/github.com/gomega/format/format.go:108 +0x3d7
    github.com/onsi/gomega/format_test.glob..func1.5.7()
        /gopath/github.com/gomega/format/format_test.go:182 +0x65
    github.com/onsi/ginkgo/internal/leafnodes.(*runner).runSync(0xc000028de0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
        /gopath/go/pkg/mod/github.com/onsi/ginkgo@v1.12.1/internal/leafnodes/runner.go:113 +0xa3
    github.com/onsi/ginkgo/internal/leafnodes.(*runner).run(0xc000028de0, 0xc, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
        /gopath/go/pkg/mod/github.com/onsi/ginkgo@v1.12.1/internal/leafnodes/runner.go:64 +0xd7
    github.com/onsi/ginkgo/internal/leafnodes.(*ItNode).Run(0xc00000eec0, 0x1434600, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
        /gopath/go/pkg/mod/github.com/onsi/ginkgo@v1.12.1/internal/leafnodes/it_node.go:26 +0x67
    github.com/onsi/ginkgo/internal/spec.(*Spec).runSample(0xc00035e870, 0x0, 0x1434600, 0xc000076900)
        /gopath/go/pkg/mod/github.com/onsi/ginkgo@v1.12.1/internal/spec/spec.go:215 +0x691
    github.com/onsi/ginkgo/internal/spec.(*Spec).Run(0xc00035e870, 0x1434600, 0xc000076900)
        /gopath/go/pkg/mod/github.com/onsi/ginkgo@v1.12.1/internal/spec/spec.go:138 +0xf2
    github.com/onsi/ginkgo/internal/specrunner.(*SpecRunner).runSpec(0xc0003272c0, 0xc00035e870, 0x8)
        /gopath/go/pkg/mod/github.com/onsi/ginkgo@v1.12.1/internal/specrunner/spec_runner.go:200 +0x111
    github.com/onsi/ginkgo/internal/specrunner.(*SpecRunner).runSpecs(0xc0003272c0, 0x1)
        /gopath/go/pkg/mod/github.com/onsi/ginkgo@v1.12.1/internal/specrunner/spec_runner.go:170 +0x127
    github.com/onsi/ginkgo/internal/specrunner.(*SpecRunner).Run(0xc0003272c0, 0xc00028d868)
        /gopath/go/pkg/mod/github.com/onsi/ginkgo@v1.12.1/internal/specrunner/spec_runner.go:66 +0x117
    github.com/onsi/ginkgo/internal/suite.(*Suite).Run(0xc000100280, 0x1dbe160, 0xc00035c000, 0x13c4138, 0xc, 0xc00031e9a0, 0x1, 0x1, 0x143bca0, 0xc000076900, ...)
        /gopath/go/pkg/mod/github.com/onsi/ginkgo@v1.12.1/internal/suite/suite.go:62 +0x426
    github.com/onsi/ginkgo.RunSpecsWithCustomReporters(0x1434ac0, 0xc00035c000, 0x13c4138, 0xc, 0xc00004c728, 0x1, 0x1, 0x100ff58)
        /gopath/go/pkg/mod/github.com/onsi/ginkgo@v1.12.1/ginkgo_dsl.go:226 +0x238
    github.com/onsi/ginkgo.RunSpecs(0x1434ac0, 0xc00035c000, 0x13c4138, 0xc, 0x135e7b80655bf)
        /gopath/go/pkg/mod/github.com/onsi/ginkgo@v1.12.1/ginkgo_dsl.go:207 +0x168
    github.com/onsi/gomega/format_test.TestFormat(0xc00035c000)
        /gopath/github.com/gomega/format/format_suite_test.go:12 +0x98
    testing.tRunner(0xc00035c000, 0x13e3308)
        /usr/local/opt/go/libexec/src/testing/testing.go:1123 +0xef
    created by testing.(*T).Run
        /usr/local/opt/go/libexec/src/testing/testing.go:1168 +0x2b3

The problem is that string.Repeat panics when count is negative which happens because the length of the message is longer than than 14:

gomega/format/format.go

Lines 106 to 108 in df70182

	tabLength := 4
	spaceFromMessageToActual := tabLength + len("<string>: ") - len(message)
	padding := strings.Repeat(" ", spaceFromMessageToActual+spacesBeforeFormattedMismatch) + "|"