Check for uppercase 'head' when updating subprojects

docs/markdown/Wrap-dependency-system-manual.md

@@ -66,7 +66,7 @@ An example wrap-git will look like this:
 ```ini
 [wrap-git]
 url = https://github.com/libfoobar/libfoobar.git
-revision = head
+revision = HEAD
 depth = 1
 ```
 
@@ -124,7 +124,7 @@ case, the directory will be copied into `subprojects/` before applying patches.
 - `url` - name of the wrap-git repository to clone. Required.
 - `revision` - name of the revision to checkout. Must be either: a
   valid value (such as a git tag) for the VCS's `checkout` command, or
-  (for git) `head` to track upstream's default branch. Required.
+  (for git) `HEAD` to track upstream's default branch. Required.
 
 ### Specific to wrap-git
 - `depth` - shallowly clone the repository to X number of commits. This saves bandwidth and disk

mesonbuild/msubprojects.py

@@ -265,7 +265,8 @@ def git_checkout(self, revision: str, create: bool = False) -> bool:
         cmd = ['checkout', '--ignore-other-worktrees']
         if create:
             cmd.append('-b')
-        cmd += [revision, '--']
+        revision_val = revision if revision.upper() != 'HEAD' else 'HEAD'
+        cmd += [revision_val, '--']
         try:
             # Stash local changes, commits can always be found back in reflog, to
             # avoid any data lost by mistake.
@@ -376,7 +377,8 @@ def update_git(self) -> bool:
                 # https://github.com/mesonbuild/meson/pull/7723#discussion_r488816189.
                 heads_refmap = '+refs/heads/*:refs/remotes/origin/*'
                 tags_refmap = '+refs/tags/*:refs/tags/*'
-                self.git_output(['fetch', '--refmap', heads_refmap, '--refmap', tags_refmap, 'origin', revision])
+                revision_val = revision if revision.upper() != 'HEAD' else 'HEAD'
+                self.git_output(['fetch', '--refmap', heads_refmap, '--refmap', tags_refmap, 'origin', revision_val])
             except GitException as e:
                 self.log('  -> Could not fetch revision', mlog.bold(revision), 'in', mlog.bold(self.repo_dir))
                 self.log(mlog.red(e.output))