Expose refspecs in git_clone_opts #56

Job	Run time
Create container: xenial	2s
Create container: bionic	2s
Create container: focal	2s
Create container: docurium	1s
Create container: bionic-x86	2s
Create container: bionic-arm32	2s
Create container: bionic-arm64	3s
Create container: centos7	1s
Create container: centos8	2s
Generate documentation	25s
Build: Linux (Xenial, GCC, OpenSSL)	28s
Build: Linux (Xenial, GCC, mbedTLS)	24s
Build: Linux (Xenial, Clang, OpenSSL)	29s
Build: Linux (Xenial, Clang, mbedTLS)	24s
Build: macOS	1s
Build: Windows (amd64, Visual Studio)	2m 17s
Build: Windows (x86, Visual Studio)	1m 27s
Build: Windows (amd64, mingw)	59s
Build: Windows (x86, mingw)	1m 0s
Build: Sanitizer (Memory)	29s
Build: Sanitizer (UndefinedBehavior)	29s
Build: Sanitizer (Thread)	28s
Build: Linux (SHA256, Xenial, Clang, OpenSSL)	24s
Build: macOS (SHA256)	1s
Build: Windows (SHA256, amd64, Visual Studio)	2m 0s
Test results	5s
	12m 7s

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