populate: follow branches and tags of git sources

A ref that consists of 4 to 40 hex digits is a commit, which populate
fetches only if the work tree lacks it, unless fetchAlways is set.  Any
other ref is a branch or tag, which populate fetches on every run,
resetting the work tree to the commit fetched.  Resetting to the name
itself resolves to a local branch, which fetching doesn't move.

1 files changed, 5 insertions, 1 deletions

diff --git a/README.md b/README.md
index d88f4e1..2784a9d 100644
--- a/README.md
+++ b/README.md
@@ -560,7 +560,11 @@ Supported attributes:
   URL of the Git repository that should be fetched.
 
 * `ref` -
-  Branch / tag / commit that should be fetched.
+  Branch / tag / commit that should be fetched.  A branch or tag gets fetched
+  on every run, so the work tree follows it.  A commit, given by its hash or a
+  prefix thereof of at least four hex digits, only gets fetched if the work
+  tree lacks it, unless `fetchAlways` is set.  So a branch or tag whose name
+  consists of hex digits only counts as a commit.
 
 * `clean.exclude` -
   List of patterns that should be excluded from Git cleaning.