guess default branch from HEAD

This is a saner alternative than hardcoding the default branch to be
"master". The add_repo() function will now check for a symbolic ref in
repo_path/HEAD. If there is a suitable one, overwrite repo->defbranch
with it. Note that you'll need to strip the newline from the file (->
len-17).

If HEAD is a symbolic link pointing directly to a branch below
refs/heads/, do a readlink() instead to find the ref name.

Signed-off-by: Julius Plenz <plenz@cis.fu-berlin.de>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgitrc.5.txt

@@ -377,7 +377,8 @@ repo.commit-filter::
 repo.defbranch::
 	The name of the default branch for this repository. If no such branch
 	exists in the repository, the first branch name (when sorted) is used
-	as default instead. Default value: "master".
+	as default instead. Default value: branch pointed to by HEAD, or
+	"master" if there is no suitable HEAD.
 
 repo.desc::
 	The value to show as repository description. Default value: none.