winhttp: use a custom user-agent if the user has set it
winhttp: use a custom user-agent if the user has set it

We also keep the "git/1.0" prefix in order to maintain compatibility
with hosters.

stewid/libgit2,saurabhsuniljain/libgit2,jeffhostetler/public_libgit2,jeffhostetler/public_libgit2,KTXSoftware/libgit2,jeffhostetler/public_libgit2,stewid/libgit2,yongthecoder/libgit2,magnus98/TEST,Corillian/libgit2,Corillian/libgit2,stewid/libgit2,magnus98/TEST,Tousiph/Demo1,yongthecoder/libgit2,magnus98/TEST,Corillian/libgit2,KTXSoftware/libgit2,joshtriplett/libgit2,KTXSoftware/libgit2,yongthecoder/libgit2,magnus98/TEST,yongthecoder/libgit2,magnus98/TEST,magnus98/TEST,Tousiph/Demo1,Tousiph/Demo1,Tousiph/Demo1,Tousiph/Demo1,leoyanggit/libgit2,stewid/libgit2,Corillian/libgit2,jeffhostetler/public_libgit2,joshtriplett/libgit2,saurabhsuniljain/libgit2,KTXSoftware/libgit2,leoyanggit/libgit2,joshtriplett/libgit2,stewid/libgit2,joshtriplett/libgit2,leoyanggit/libgit2,leoyanggit/libgit2,joshtriplett/libgit2,jeffhostetler/public_libgit2,yongthecoder/libgit2,joshtriplett/libgit2,leoyanggit/libgit2,yongthecoder/libgit2,jeffhostetler/public_libgit2,Corillian/libgit2,saurabhsuniljain/libgit2,Tousiph/Demo1,leoyanggit/libgit2,saurabhsuniljain/libgit2,saurabhsuniljain/libgit2,stewid/libgit2,KTXSoftware/libgit2,Corillian/libgit2,saurabhsuniljain/libgit2,KTXSoftware/libgit2