Merge branch 'git_branch' into 'master'

Ensure GIT_BRANCH non-empty when not on a branch

If HEAD is detached e.g. during `git bisect`, GIT_BRANCH ends up empty, which causes an error at runtime. This change grabs the description from `git branch`, which is either the branch name or something like `(HEAD detached at <hash>)` on recent versions or `(no branch)` in older versions. Works with BSD and GNU sed, and git >= 1.0.0b (at least) although detached HEAD only introduced in 1.5.0.

See merge request !47

src/Makefile.am

@@ -63,10 +63,10 @@ libswiftsim_mpi_la_SHORTNAME = mpi
 version.h: version.h.in $(AM_SOURCES) $(include_HEADERS) $(noinst_HEADERS)
 	if test "X$(GIT_CMD)" != "X"; then \
 	    GIT_REVISION=`$(GIT_CMD) describe --abbrev=8  --always --tags --dirty`; \
-	    GIT_BRANCH=`$(GIT_CMD) symbolic-ref HEAD| sed 's,.*/,,'`; \
+	    GIT_BRANCH=`$(GIT_CMD) branch | sed -n 's/^\* \(.*\)/\1/p'`; \
 	    sed -e "s,@PACKAGE_VERSION\@,$(PACKAGE_VERSION)," \
 	        -e "s,@GIT_REVISION\@,$${GIT_REVISION}," \
-	        -e "s,@GIT_BRANCH\@,$${GIT_BRANCH}," version.h.in > version.h; \
+	        -e "s|@GIT_BRANCH\@|$${GIT_BRANCH}|" version.h.in > version.h; \
 	else \
 	    if test ! -f version.h; then \
 	        sed -e "s,@PACKAGE_VERSION\@,$(PACKAGE_VERSION)," \