libgit2: update to 0.21.0 with thread safe support
vala: update to version 0.25.1
Merge pull request #92 from nacho/wip/libgit2

libgit2: update to 0.21.0 with thread safe support
Merge pull request #93 from nacho/wip/vala

vala: update to version 0.25.1
gtk3: Fix copying file.