Build fails on `master` because from/to revisions are the same

[asgrim]

from https://travis-ci.org/Roave/ApiCompare/jobs/372001598 :

$ bin/roave-backward-compatibility-check --from=master --to=HEAD
Comparing from 4b9138b628351653a8064b81bf2ee9ef062e0545 to 4b9138b628351653a8064b81bf2ee9ef062e0545...
In Process.php line 222:
                                                                               
  The command "'git' 'clone' '/home/travis/build/Roave/ApiCompare' '/tmp/api-  
  compare-4b9138b628351653a8064b81bf2ee9ef062e0545'" failed.                   
                                                                               
  Exit Code: 128(Invalid exit argument)                                        
                                                                               
  Working directory: /home/travis/build/Roave/ApiCompare                       
                                                                               
  Output:                                                                      
  ================                                                             
                                                                               
                                                                               
  Error Output:                                                                
  ================                                                             
  fatal: destination path '/tmp/api-compare-4b9138b628351653a8064b81bf2ee9ef0  
  62e0545' already exists and is not an empty directory.                       
                                                                               
roave-backwards-compatibility-check:assert-backwards-compatible [--from [FROM]] [--to TO] [--format [FORMAT]] [--] [<sources-path>]
The command "bin/roave-backward-compatibility-check --from=master --to=HEAD" exited with 1.

[Ocramius]

Question: do we even need the SHA1 to be part of the path? Would a tempnam() not suffice?

[asgrim]

It's useful for referencing, I'll just wrap it in uniqid I reckon