Check for uppercase 'head' when updating subprojects (fixes #12730)

[andy5995]

This change may be more robust. I get these errors when using head for the value of revision in a wrap file and using 'checkout' or 'update' with the meson subprojects option:

$ meson subprojects checkout
Checkout head in decimal...
  -> Could not checkout head in subprojects/decimal
fatal: invalid reference: head
Git command failed: ['/usr/bin/git', 'checkout', '--ignore-other-worktrees', 'head', '--']

$ meson subprojects update
Updating decimal...
  -> Could not fetch revision head in subprojects/decimal
fatal: couldn't find remote ref head
Git command failed: ['/usr/bin/git', 'fetch', '--refmap', '+refs/heads/*:refs/remotes/origin/*', '--refmap', '+refs/tags/*:refs/tags/*', 'origin', 'head']

When I change the revision value to 'HEAD', there's no problem:

$ meson subprojects update
Updating decimal...
-> 0d27065 (grafted, HEAD -> main, origin/main, origin/HEAD) Update installation document to state this project has now git submodules. [semihc]

EDIT:

fixes #12730

[andy5995]

So after realizing this was more of a bug than a doc issue, I was able to find some of the code. I left the HEAD uppercase in the docs for now. I figured git is case-sensitive about HEAD and probably best practices would indicate using "HEAD" is a better habit. But of course I'll change the docs back if you prefer.

[andy5995]

I believe this is a fix for #12730