Hide commit signature information when fetching commit hash
If someone configures git to always show a commit’s signing information,
this breaks the definition of GitVersion in revision.h:
```
#ifndef ASSIMP_REVISION_H_INC
#define ASSIMP_REVISION_H_INC
#define GitVersion 0xgpg: Signature faite le Fri 15 Jun 2018 20:39:53 CEST
gpg: avec la clef RSA 4AEE18F83AFDEB23
gpg: Impossible de vérifier la signature : Pas de clef publique
878b4b2c
#define GitBranch "master"
#endif // ASSIMP_REVISION_H_INC
```
This can be avoided by passing the `--no-show-signature` to the `git
log` command for retrieving the hash.
Pierre Moreau