Don't read revision hash from the git repository.

--- a/Makefile
+++ b/Makefile
@@ -18,11 +18,7 @@
 	cp .version zsh-syntax-highlighting.zsh $(SHARE_DIR)
 	cp COPYING.md README.md changelog.md $(DOC_DIR)
 	sed -e '1s/ .*//' -e '/^\[build-status-[a-z]*\]: /d' < README.md > $(DOC_DIR)/README.md
-	if [ x"true" = x"`git rev-parse --is-inside-work-tree 2>/dev/null`" ]; then \
-		git rev-parse HEAD; \
-	else \
-		cat .revision-hash; \
-	fi > $(SHARE_DIR)/.revision-hash
+	cp .revision-hash $(SHARE_DIR)
 	:
 # The [ -e ] check below is to because sh evaluates this with (the moral
 # equivalent of) NONOMATCH in effect, and highlighters/*.zsh has no matches.