cmake: default to using SHA1DC

Upstream git.git has changed their default SHA1 implementation to the
collision-detection algorithm SHA1DC in commit e6b07da27 (Makefile: make
DC_SHA1 the default, 2017-03-17). To match upstream, align ourselves and
switch over to SHA1DC by default.

CMakeLists.txt

@@ -42,7 +42,7 @@ OPTION( PROFILE				"Generate profiling information"		OFF )
 OPTION( ENABLE_TRACE		"Enables tracing support"				OFF )
 OPTION( LIBGIT2_FILENAME	"Name of the produced binary"			OFF )
 
-OPTION( USE_SHA1DC			"Use SHA-1 with collision detection"	OFF )
+OPTION( USE_SHA1DC			"Use SHA-1 with collision detection"	ON )
 OPTION( USE_SSH				"Link with libssh to enable SSH support" ON )
 OPTION( USE_HTTPS			"Enable HTTPS support. Can be set to a specific backend"	ON )
 OPTION( USE_GSSAPI			"Link with libgssapi for SPNEGO auth"   OFF )