Diff: Do not try to calculate an oid for a GITLINK.
Diff: Do not try to calculate an oid for a GITLINK.

We don't have anything useful that we could do with that oid anyway (We
need to query the submodule for the HEAD commit instead).

Without this, the following code creates the error "Failed to read
descriptor: Is a directory" when run against the submod2 test-case:

    const char* oidstr = "873585b94bdeabccea991ea5e3ec1a277895b698";
    git_tree* tree = resolve_commit_oid_to_tree(g_repo, oidstr);
    git_diff_list* diff = NULL;
    cl_assert(tree);
    cl_git_pass(git_diff_workdir_to_tree(g_repo, NULL, tree, &diff));

oaastest/libgit2,Aorjoa/libgit2_maked_lib,mcanthony/libgit2,iankronquist/libgit2,kissthink/libgit2,falqas/libgit2,rcorre/libgit2,oaastest/libgit2,since2014/libgit2,swisspol/DEMO-libgit2,evhan/libgit2,mrksrm/Mingijura,since2014/libgit2,swisspol/DEMO-libgit2,skabel/manguse,stewid/libgit2,mcanthony/libgit2,iankronquist/libgit2,spraints/libgit2,magnus98/TEST,ardumont/libgit2,t0xicCode/libgit2,evhan/libgit2,sim0629/libgit2,chiayolin/libgit2,KTXSoftware/libgit2,leoyanggit/libgit2,KTXSoftware/libgit2,jamieleecool/ptest,kenprice/libgit2,KTXSoftware/libgit2,yongthecoder/libgit2,mrksrm/Mingijura,Tousiph/Demo1,saurabhsuniljain/libgit2,skabel/manguse,oaastest/libgit2,jflesch/libgit2-mariadb,sim0629/libgit2,jflesch/libgit2-mariadb,KTXSoftware/libgit2,sim0629/libgit2,iankronquist/libgit2,chiayolin/libgit2,yongthecoder/libgit2,nokiddin/libgit2,MrHacky/libgit2,nacho/libgit2,claudelee/libgit2,spraints/libgit2,since2014/libgit2,rcorre/libgit2,kissthink/libgit2,saurabhsuniljain/libgit2,Corillian/libgit2,spraints/libgit2,Corillian/libgit2,rcorre/libgit2,linquize/libgit2,whoisj/libgit2,swisspol/DEMO-libgit2,swisspol/DEMO-libgit2,chiayolin/libgit2,kissthink/libgit2,kissthink/libgit2,jeffhostetler/public_libgit2,claudelee/libgit2,Corillian/libgit2,jeffhostetler/public_libgit2,oaastest/libgit2,chiayolin/libgit2,stewid/libgit2,joshtriplett/libgit2,Snazz2001/libgit2,yosefhackmon/libgit2,MrHacky/libgit2,dleehr/libgit2,sim0629/libgit2,yongthecoder/libgit2,JIghtuse/libgit2,kissthink/libgit2,amyvmiwei/libgit2,leoyanggit/libgit2,chiayolin/libgit2,yosefhackmon/libgit2,Snazz2001/libgit2,raybrad/libit2,yosefhackmon/libgit2,linquize/libgit2,nokiddin/libgit2,falqas/libgit2,magnus98/TEST,Snazz2001/libgit2,raybrad/libit2,since2014/libgit2,raybrad/libit2,Corillian/libgit2,maxiaoqian/libgit2,Tousiph/Demo1,iankronquist/libgit2,falqas/libgit2,nokiddin/libgit2,rcorre/libgit2,stewid/libgit2,saurabhsuniljain/libgit2,jflesch/libgit2-mariadb,oaastest/libgit2,amyvmiwei/libgit2,falqas/libgit2,swisspol/DEMO-libgit2,jeffhostetler/public_libgit2,claudelee/libgit2,leoyanggit/libgit2,mhp/libgit2,linquize/libgit2,mrksrm/Mingijura,joshtriplett/libgit2,zodiac/libgit2.js,MrHacky/libgit2,leoyanggit/libgit2,kenprice/libgit2,yongthecoder/libgit2,ardumont/libgit2,mrksrm/Mingijura,mcanthony/libgit2,yongthecoder/libgit2,spraints/libgit2,MrHacky/libgit2,Snazz2001/libgit2,amyvmiwei/libgit2,evhan/libgit2,yosefhackmon/libgit2,claudelee/libgit2,JIghtuse/libgit2,zodiac/libgit2.js,ardumont/libgit2,stewid/libgit2,whoisj/libgit2,leoyanggit/libgit2,saurabhsuniljain/libgit2,JIghtuse/libgit2,JIghtuse/libgit2,chiayolin/libgit2,Snazz2001/libgit2,whoisj/libgit2,Aorjoa/libgit2_maked_lib,mingyaaaa/libgit2,Corillian/libgit2,yosefhackmon/libgit2,jeffhostetler/public_libgit2,whoisj/libgit2,amyvmiwei/libgit2,iankronquist/libgit2,t0xicCode/libgit2,nacho/libgit2,since2014/libgit2,magnus98/TEST,mingyaaaa/libgit2,leoyanggit/libgit2,linquize/libgit2,dleehr/libgit2,jeffhostetler/public_libgit2,mingyaaaa/libgit2,magnus98/TEST,KTXSoftware/libgit2,sim0629/libgit2,mcanthony/libgit2,whoisj/libgit2,jflesch/libgit2-mariadb,jamieleecool/ptest,mcanthony/libgit2,nacho/libgit2,sygool/libgit2,sim0629/libgit2,jflesch/libgit2-mariadb,raybrad/libit2,maxiaoqian/libgit2,dleehr/libgit2,spraints/libgit2,skabel/manguse,kenprice/libgit2,t0xicCode/libgit2,magnus98/TEST,dleehr/libgit2,joshtriplett/libgit2,evhan/libgit2,skabel/manguse,maxiaoqian/libgit2,JIghtuse/libgit2,ardumont/libgit2,whoisj/libgit2,sygool/libgit2,magnus98/TEST,JIghtuse/libgit2,claudelee/libgit2,Aorjoa/libgit2_maked_lib,t0xicCode/libgit2,swisspol/DEMO-libgit2,zodiac/libgit2.js,sygool/libgit2,Tousiph/Demo1,kenprice/libgit2,yongthecoder/libgit2,saurabhsuniljain/libgit2,oaastest/libgit2,Tousiph/Demo1,mhp/libgit2,MrHacky/libgit2,skabel/manguse,kenprice/libgit2,mingyaaaa/libgit2,ardumont/libgit2,t0xicCode/libgit2,Aorjoa/libgit2_maked_lib,yosefhackmon/libgit2,mingyaaaa/libgit2,skabel/manguse,dleehr/libgit2,nokiddin/libgit2,nokiddin/libgit2,mhp/libgit2,nokiddin/libgit2,mcanthony/libgit2,raybrad/libit2,mhp/libgit2,saurabhsuniljain/libgit2,kenprice/libgit2,Aorjoa/libgit2_maked_lib,since2014/libgit2,maxiaoqian/libgit2,spraints/libgit2,jamieleecool/ptest,joshtriplett/libgit2,claudelee/libgit2,nacho/libgit2,kissthink/libgit2,sygool/libgit2,jeffhostetler/public_libgit2,linquize/libgit2,amyvmiwei/libgit2,mhp/libgit2,Snazz2001/libgit2,Tousiph/Demo1,mrksrm/Mingijura,sygool/libgit2,jflesch/libgit2-mariadb,joshtriplett/libgit2,amyvmiwei/libgit2,maxiaoqian/libgit2,rcorre/libgit2,Corillian/libgit2,stewid/libgit2,jamieleecool/ptest,stewid/libgit2,ardumont/libgit2,rcorre/libgit2,falqas/libgit2,sygool/libgit2,KTXSoftware/libgit2,dleehr/libgit2,MrHacky/libgit2,mrksrm/Mingijura,maxiaoqian/libgit2,Tousiph/Demo1,iankronquist/libgit2,t0xicCode/libgit2,mingyaaaa/libgit2,falqas/libgit2,linquize/libgit2,zodiac/libgit2.js,zodiac/libgit2.js,mhp/libgit2,joshtriplett/libgit2