BUG: sha512 file must exist before complaining that it hasn't been updated
BUG: sha512 file must exist before complaining that it hasn't been updated

When updating tests files, MD5 content links are created by CMake ExternalData
module. If the MD5 is updated, `pre-commit` verifies that the SHA512 was also
updated. However, if the SHA512 file does not exist, `pre-commit` should not
complain that it hasn't been updated. This case could happen if a user adds
a new data file for which only the MD5 file has been computed, and then
modifies it (or amends the first commit in which the file has been added).

Change-Id: I7aa8abd4f9ae0464f64fbcad413ce9b2e2aeb55a

BRAINSia/ITK,hjmjohnson/ITK,malaterre/ITK,spinicist/ITK,stnava/ITK,malaterre/ITK,thewtex/ITK,spinicist/ITK,Kitware/ITK,vfonov/ITK,spinicist/ITK,stnava/ITK,InsightSoftwareConsortium/ITK,thewtex/ITK,thewtex/ITK,richardbeare/ITK,LucasGandel/ITK,spinicist/ITK,blowekamp/ITK,richardbeare/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,stnava/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,thewtex/ITK,fbudin69500/ITK,LucasGandel/ITK,Kitware/ITK,blowekamp/ITK,richardbeare/ITK,hjmjohnson/ITK,fbudin69500/ITK,stnava/ITK,blowekamp/ITK,LucasGandel/ITK,Kitware/ITK,thewtex/ITK,thewtex/ITK,fbudin69500/ITK,fbudin69500/ITK,blowekamp/ITK,hjmjohnson/ITK,Kitware/ITK,spinicist/ITK,BRAINSia/ITK,vfonov/ITK,richardbeare/ITK,Kitware/ITK,richardbeare/ITK,blowekamp/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,stnava/ITK,Kitware/ITK,LucasGandel/ITK,hjmjohnson/ITK,malaterre/ITK,BRAINSia/ITK,blowekamp/ITK,stnava/ITK,vfonov/ITK,blowekamp/ITK,malaterre/ITK,BRAINSia/ITK,vfonov/ITK,BRAINSia/ITK,spinicist/ITK,vfonov/ITK,BRAINSia/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,stnava/ITK,spinicist/ITK,spinicist/ITK,LucasGandel/ITK,malaterre/ITK,thewtex/ITK,richardbeare/ITK,vfonov/ITK,fbudin69500/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,fbudin69500/ITK,spinicist/ITK,LucasGandel/ITK,hjmjohnson/ITK,fbudin69500/ITK,vfonov/ITK,stnava/ITK,fbudin69500/ITK,richardbeare/ITK,malaterre/ITK,stnava/ITK,LucasGandel/ITK,vfonov/ITK,BRAINSia/ITK,malaterre/ITK,blowekamp/ITK