Fix compilation if git signatures shown by default (#488)

Without this change then the presence of:
  [log]
    showSignature = True

in the local ~/.gitconfig file breaks compilation of CoreNEURON.

Author: olupton

CMake/GitRevision.cmake

@@ -14,13 +14,13 @@ find_package(Git)
 if(GIT_FOUND)
   # get last commit sha1
   execute_process(
-    COMMAND ${GIT_EXECUTABLE} log -1 --format=%h
+    COMMAND ${GIT_EXECUTABLE} -c log.showSignature=false log -1 --format=%h
     WORKING_DIRECTORY ${CORENEURON_PROJECT_SOURCE_DIR}
     OUTPUT_VARIABLE GIT_REVISION_SHA1
     ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE)
   # get last commit date
   execute_process(
-    COMMAND ${GIT_EXECUTABLE} show -s --format=%ci
+    COMMAND ${GIT_EXECUTABLE} -c log.showSignature=false show -s --format=%ci
     WORKING_DIRECTORY ${CORENEURON_PROJECT_SOURCE_DIR}
     OUTPUT_VARIABLE GIT_REVISION_DATE
     ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE)