devel/gitg: unbreak by updating to 3.26.0

Current version of libgit2 and libgit2-glib in ports were not supported by
gitg 3.24, so update this port to 3.26 to fix the breakage.

PR: 221783
Submitted by: rozhuk.im@gmail.com (original patch)

romain (intermediate patch)
Hiroki Tagato <tagattie@yandex.com> (committed patch)

With hat: gnome