fix: be more helpful on bad args

[alexeldeib]

If you manually pass an empty string ("") for either argument, main will panic. I found this out because shellcheck likes double quotes around variables and I ran apidiff with an empty variable.

Before:

ace@ace-home-vm:~/go/src/github.com/joelanford/go-apidiff$ ../apidiff ""
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x96b2bd]

ace@ace-home-vm:~/go/src/github.com/joelanford/go-apidiff$ ../apidiff "HEAD" ""
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x96b2bd]

After:

ace@ace-home-vm:~/go/src/github.com/joelanford/go-apidiff$ ../apidiff ""
Error: oldCommit should not be empty
Usage:
  go-apidiff <oldCommit> [newCommit] [flags]
...

ace@ace-home-vm:~/go/src/github.com/joelanford/go-apidiff$ ../apidiff "HEAD" ""
Error: if provided, newCommit should not be empty
Usage:
  go-apidiff <oldCommit> [newCommit] [flags]
...

[alexeldeib]

btw, awesome tool

[joelanford]

Looks good! Thanks!