Correctly determine git SHA if libdivecomputer is a git submodule

In the case of a submodule, the .git file is a text file pointing to the
correct module in the parent's .git folder. The git rev-parse works
correctly in both cases.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

src/Makefile.am

@@ -100,7 +100,7 @@ libdivecomputer.lo: revision.h
 
 FORCE:
 $(top_srcdir)/revision: FORCE
-	@if (test -d $(top_srcdir)/.git && cd $(top_srcdir) \
+	@if (test -e $(top_srcdir)/.git && cd $(top_srcdir) \
 	     && git rev-parse --verify HEAD) > revision-t 2>/dev/null \
 	  && ! cmp -s revision-t $@; then \
 	  mv -f revision-t $@; \