version: don't have a third version number form for xcode

Our primary version format is git describe --long --abbrev=9.

Our Apple scheme is:
    (major+100).minor.(patch*10,000+gitDescribeCommits).

This CL gets rid of the third, which was:
    major.minor.(patch*10,000+gitDescribeCommits).

Now the "About" box in the macOS app shows the same version that we
show on pkgs.tailscale.com, userz, changelog, etc.

This will be more important once/if we get standalone DMG downloads
for macOS on pkgs.tailscale.com.

Fixes tailscale/corp#364

version/mkversion.sh

@@ -61,6 +61,14 @@ xcode() {
     patch=$(echo "$ver" | cut -f3 -d.)
     changecount=$(echo "$ver" | cut -f4 -d.)
 
+    # name should be like git-describe, but without the git suffix or
+    # the leading v. For example, for git-describe of
+    # "v0.100.0-15-gce1b52bb7" we want:
+    #
+    #   VERSION_NAME = 0.100.0-15
+    #   VERSION_ID = 100.100.15
+    name=$(echo "$describe" | sed -e 's/^v//' | sed -e 's/-g.*//' | sed -e 's/-0$//')
+
     # Apple version numbers must be major.minor.patch. We have 4 fields
     # because we need major.minor.patch for go module compatibility, and
     # changecount for automatic version numbering of unstable builds. To
@@ -68,8 +76,8 @@ xcode() {
     patch=$((patch*10000 + changecount))
 
     # CFBundleShortVersionString: the "short name" used in the App Store.
-    # eg. 0.92.98
-    echo "VERSION_NAME = $major.$minor.$patch"
+    # e.g. 0.92.98
+    echo "VERSION_NAME = \"$name\""
     # CFBundleVersion: the build number. Needs to be 3 numeric sections
     # that increment for each release according to SemVer rules.
     #