Win: quote equal sign in command line

`cmd.exe` splits the command line also by equal signs, not only by
space characters.

Author: nobu

common.mk

@@ -46,7 +46,7 @@ RUN_OPTS      = --disable-gems
 
 GIT_IN_SRC    = $(GIT) -C $(srcdir)
 GIT_LOG       = $(GIT_IN_SRC) log --no-show-signature
-GIT_LOG_FORMAT = $(GIT_LOG) --pretty=format:
+GIT_LOG_FORMAT = $(GIT_LOG) "--pretty=format:"
 
 # GITPULLOPTIONS = --no-tags