appveyor: Bump shallow clone depth.

To prevent build failures when a large patch series is committed, like
happened in https://ci.appveyor.com/project/jrfonseca-fdo/mesa/build/322
due to 10 commits between dac2964f3e and
6f428328d3 where submitted before the
build slave started the git clone.

100 commits should be bigger than any patch series seen in practice, and
it takes practically the same time to download as 5 commits.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>

appveyor.yml

@@ -6,7 +6,7 @@
 # - Select Git and fill in the Git clone URL
 # - Setup a Git hook as explained in
 #   https://github.com/appveyor/webhooks#installing-git-hook
-# - Check 'Settings > General > Skip branches without appveyor'
+# - Check 'Settings > General > Skip branches without appveyor.yml'
 # - Check 'Settings > General > Rolling builds'
 # - Setup the global or project notifications to your liking
 #
@@ -24,7 +24,14 @@ branches:
   except:
   - /^travis.*$/
 
-clone_depth: 5
+# Don't download the full Mesa history to speed up cloning.  However the clone
+# depth must not be too small, otherwise builds might fail when lots of patches
+# are committed in succession, because the desired commit is not found on the
+# truncated history.
+#
+# See also:
+# - https://www.appveyor.com/blog/2014/06/04/shallow-clone-for-git-repositories
+clone_depth: 100
 
 cache:
 - win_flex_bison-2.4.5.zip