Create a reference tag if none exists.

Author: paulp

tools/get-scala-revision

@@ -10,12 +10,21 @@
 
 [[ $# -eq 0 ]] || cd "$1"
 
+ensure_tag () {
+  # v2.10.0-M1
+  fallback=58cb15c40d
+  [[ -n $(git tag -l v2.10.0-M1) ]] || {
+    git tag -a -m "generated by get-scala-revision" v2.10.0-M1 $fallback
+  }
+}
+ensure_tag
+
 # the closest tag, obtained separately because we have to
 # reconstruct the string around the padded distance.
-tag=$(git describe --abbrev=0)
+tag=$(git describe --match 'v2*' --abbrev=0)
 
 # the full string - padding correctness depends on abbrev=10.
-described=$(git describe --abbrev=10 --always --tags)
+described=$(git describe --match 'v2*' --abbrev=10)
 suffix="${described##${tag}-}"
 counter=$(echo $suffix | cut -d - -f 1)
 hash=$(echo $suffix | cut -d - -f 2)