devel/libgit2-glib: Unbreak with libgit2 0.27.

The fix is two-fold:

• Adjust the path for the sed call in devel/libgit2 so that we leave out openssl from libgit2.pc's Requires.private line, otherwise libgit2-glib will look for openssl.pc, which does not exist when OpenSSL from base is used.

• Reset libgit2-glib back to 0.26, and import a patch both Arch Linux and openSUSE have been carrying to fix the build with libgit2 0.27. PORTEPOCH was not set because there is no libgit2-glib 0.27 and no package could have possibly been built after r469930.

Approved by: mfechner, gnome (kwm)
Differential Revision: https://reviews.freebsd.org/D15437