Make gowork-snapshot support repositories in detached head

When git repositories are in detached head, `git symbolic-ref` cannot be used.

In this case, `gowork-snapshot` outputs `fatal: ref origin is not a symbolic ref` and create a buildout profile with empty revisions.

After these changes, we try to get from a remote named origin in this case. If this did not work, we output the problematic repository.

I added some minor "cleanups" at the same time.

/reviewed-on nexedi/slapos!268
/reviewed-by kirr

component/golang/gowork-snapshot

@@ -25,9 +25,16 @@ gogit_list() {
 # git_upstream_url <repo>   - show current branch upstream URL
 git_upstream_url() {
     repo=$1
-    head="`git -C $repo symbolic-ref --short HEAD`"             # current branch - e.g. "t"
-    remote="`git -C $repo config --get branch.$head.remote`"    # upstream name, e.g. "kirr"
+    head="`git -C $repo symbolic-ref -q --short HEAD`"            # current branch - e.g. "master"
+    if [ -z $head ] ; then
+        remote="origin" # If we are in detached HEAD, assume the upstream name is origin
+    else
+        remote="`git -C $repo config --get branch.$head.remote`"  # upstream name, e.g. "origin"
+    fi
     url="`git -C $repo config --get remote.$remote.url`"          # upstream URL
+    if [ -z $url ] ; then
+        echo "ERROR: Failed to guess upstream url from $repo" 1>&2
+    fi
     echo "$url"
 }