We currently use `FILE(GLOB ...)` in most places to find source and
header files. This is problematic in that the order of files returned
depends on the operating system's directory iteration order and may thus
not be deterministic. As a result, we link object files in unspecified
order, which may cause the linker to emit different code across runs.

Fix this issue by sorting all code used as input to the libgit2 library
to improve the reliability of reproducible builds.