message: How to regen LIBGIT2_SOURCE for new libgit2
diff: -LIBGIT2_FILES := $(wildcard ext/libgit2/src/*.c ext/libgit2/src/*/*.c)
  -LIBGIT2_SOURCE:=$(LIBGIT2_FILES:ext/libgit2/%=%)
  +#
  +# When upgrading libgit2 to a new version,
  +# uncomment these lines and look for missing files between the output and
  +# the contents of the LIBGIT2_SOURCE variable below.
  +#
  +#LIBGIT2_FILES := $(wildcard ext/libgit2/src/*.c ext/libgit2/src/*/*.c)
  +#LIBGIT2_SOURCE:=$(LIBGIT2_FILES:ext/libgit2/%=%)
  +#$(warning LIBGIT2_SOURCE=$(LIBGIT2_SOURCE))
  LIBGIT2_SOURCE:= \
  src/annotated_commit.c \
  