I’ve got this message on Ubuntu 12.04. Other servers seemed to work fine.

root@server:/tmp# git clone https://git.kolab.org/diffusion/RPK/roundcubemail-plugins-kolab.git
Cloning into 'roundcubemail-plugins-kolab'...
error: server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none while accessing https://git.kolab.org/diffusion/RPK/roundcubemail-plugins-kolab.git/info/refs
fatal: HTTP request failed

The fix:
Append CA root of that cert to file: /etc/ssl/certs/ca-certificates.crt

OR
just to disable sslverify for git globally (it may be reenabled afterwards):

root@server:/tmp# git config --global http.sslverify false

And everything is back to normal:

root@server:/tmp# git clone https://git.kolab.org/diffusion/RPK/roundcubemail-plugins-kolab.git
Cloning into 'roundcubemail-plugins-kolab'...
remote: Counting objects: 28075, done.
remote: Compressing objects: 100% (8146/8146), done.
remote: Total 28075 (delta 19515), reused 26099 (delta 18003)
Receiving objects: 100% (28075/28075), 7.53 MiB | 748 KiB/s, done.
Resolving deltas: 100% (19515/19515), done.