Does not work on RHEL 6 (with git 1.7.1) as "git rev-list" supports "--count" only since git 1.7.2

[xtaran]

If I use zsh-vcs-prompt (current master at 68d68a5) on Red Hat Enterprise Linux (RHEL) 6, I always get a prompt like this:

usage: git rev-list [OPTION] <commit-id>... [ -- paths... ]                                                   
  limiting output:
    --max-count=nr
    --max-age=epoch
    --min-age=epoch
    --sparse
    --no-merges
    --remove-empty
    --all
    --branches
    --tags
    --remotes
    --stdin
    --quiet
  ordering output:
    --topo-order
    --date-order
    --reverse
  formatting output:
    --parents
    --children
    --objects | --objects-edge
    --unpacked
    --header | --pretty
    --abbrev=nr | --no-abbrev
    --abbrev-commit
    --left-right
  special purpose:
    --bisect
    --bisect-vars
    --bisect-all
!8206 Z16 ?0 L1 ssh …@…:~/.zsh/zsh-vcs-prompt (pts/3 zsh 4.3.11 RHEL 6.10) 14:29:28 (git) [master|+1]
~/.zsh/zsh-vcs-prompt →

The last two lines are the expected prompt, the lines above are triggered by line 174 of vcsstatus.sh.

Reason is that RHEL 6 ships with git 1.7.1 and the option --count was only introduced just one release afterwards with git 1.7.2. :-(

Interestingly the usage of --count in gitstatus-fast.py line 133 doesn't seem to cause such issues for me. Maybe I never had a case where this was used, so I'd expect that it needs the same fix.

It's annoying, but I currently have no idea how this could be easily fixed without dropping functionality.