travis-ci: run previously failed tests first, then slowest to fastest

The Travis-CI machines are in a clean state in the beginning of every run
(transient by default). Use the Travis-CI cache feature to make the prove
state persistent across consecutive Travis-CI runs on the same branch.
This allows to run previously failed tests first and run remaining tests
in slowest to fastest order. As a result it is less likely that Travis-CI
needs to wait for a single test at the end which speeds up the test suite
execution by ~2 min.

Travis-CI can only cache entire directories. Prove stores the .prove file
always in the t/ directory but we don't want to cache the entire t/ directory.
Therefore we create a symlink from $HOME/travis-cache/.prove to t/.prove and
cache the $HOME/travis-cache directory.

Unfortunately the cache feature is only available (for free) on the
Travis-CI Linux environment.
Suggested-by: NJeff King <peff@peff.net>
Signed-off-by: NLars Schneider <larsxschneider@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

.travis.yml

 language: c
 
+cache:
+  directories:
+    - $HOME/travis-cache
+
 os:
   - linux
   - osx
@@ -18,7 +22,7 @@ env:
     - P4_VERSION="15.2"
     - GIT_LFS_VERSION="1.1.0"
     - DEFAULT_TEST_TARGET=prove
-    - GIT_PROVE_OPTS="--timer --jobs 3"
+    - GIT_PROVE_OPTS="--timer --jobs 3 --state=failed,slow,save"
     - GIT_TEST_OPTS="--verbose --tee"
     - CFLAGS="-g -O2 -Wall -Werror"
     - GIT_TEST_CLONE_2GB=YesPlease
@@ -67,6 +71,8 @@ before_install:
     p4 -V | grep Rev.;
     echo "$(tput setaf 6)Git-LFS Version$(tput sgr0)";
     git-lfs version;
+    mkdir -p $HOME/travis-cache;
+    ln -s $HOME/travis-cache/.prove t/.prove;
 
 before_script: make --jobs=2