0.13.0 doesn't like merging branches with slashes

[imbaczek]

goroutine 1 [running]:
github.com/ejoffe/spr/git/realgit.(*gitcmd).MustGit(0x14000125080, {0x140006f4000, 0x217}, 0x0)
        /Users/runner/work/spr/spr/git/realgit/realcmd.go:44 +0x84
github.com/ejoffe/spr/spr.(*stackediff).syncCommitStackToGitHub(0x1400012b1a0, {0x1011204b0, 0x1400011a000}, {0x14000168b40, 0x5, 0x8}, 0x140000a2280)
        /Users/runner/work/spr/spr/spr/spr.go:532 +0x29c
github.com/ejoffe/spr/spr.(*stackediff).UpdatePullRequests(0x1400012b1a0, {0x1011204b0, 0x1400011a000}, {0x0, 0x0, 0x0}, 0x0)
        /Users/runner/work/spr/spr/spr/spr.go:160 +0x520
main.main.func5(0x1400012d600)
        /Users/runner/work/spr/spr/cmd/spr/main.go:187 +0xf0
github.com/urfave/cli/v2.(*Command).Run(0x140002207e0, 0x1400012d440)
        /Users/runner/go/pkg/mod/github.com/urfave/cli/v2@v2.3.0/command.go:163 +0x634
github.com/urfave/cli/v2.(*App).RunContext(0x140001a11e0, {0x1011204b0, 0x1400011a000}, {0x14000130000, 0x2, 0x2})
        /Users/runner/go/pkg/mod/github.com/urfave/cli/v2@v2.3.0/app.go:313 +0x760
github.com/urfave/cli/v2.(*App).Run(...)
        /Users/runner/go/pkg/mod/github.com/urfave/cli/v2@v2.3.0/app.go:224
main.main()
        /Users/runner/work/spr/spr/cmd/spr/main.go:224 +0x150c