cgit: Fix guessing default branch

Even when HEAD is set to "main" in a repository, cgit still uses the
"master" branch to detect the age of the last commit.

Example:

    $ cat repositories/openwrt/openwrt.git/HEAD
    ref: refs/heads/main

    $ stat repositories/openwrt/openwrt.git/refs/heads/master
    Modify: 2024-02-22 23:00:01.285731731 +0000

    $ stat repositories/openwrt/openwrt.git/refs/heads/main
    Modify: 2026-02-27 15:33:49.506017172 +0000

In this situation, cgit displays "25 months" for this repository on the index page.

I guess the internal format for HEAD has changed at some point, so it
needs to be taken into account when parsing it in cgit.

Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>