t4201: make use of abbreviation in the test more robust

The test for '--abbrev' in t4201-shortlog.sh assumes that the commits
generated in the test can always be uniquely abbreviated to 5 hex digits
but this is not always the case. If you were unlucky and happened to run
the test at (say) Thu Jun 22 03:04:49 2017 +0000, you would find that
the first commit generated would collide with a tree object created
later in the same test.

This can be simulated in the version of t4201-shortlog.sh prior to this
commit by setting GIT_COMMITTER_DATE and GIT_AUTHOR_DATE to 1498100689
after sourcing test-lib.sh.

Change the test to test --abbrev=35 instead of --abbrev=5 to almost
completely avoid the possibility of a partial collision and add a call
to test_tick in the setup to make the test repeatable (the latter alone
is sufficient to make it robust enough).

Signed-off-by: Charles Bailey <cbailey32@bloomberg.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4201-shortlog.sh

@@ -9,6 +9,7 @@ test_description='git shortlog
 . ./test-lib.sh
 
 test_expect_success 'setup' '
+	test_tick &&
 	echo 1 >a1 &&
 	git add a1 &&
 	tree=$(git write-tree) &&
@@ -59,7 +60,7 @@ fuzz() {
 	file=$1 &&
 	sed "
 			s/$_x40/OBJECT_NAME/g
-			s/$_x05/OBJID/g
+			s/$_x35/OBJID/g
 			s/^ \{6\}[CTa].*/      SUBJECT/g
 			s/^ \{8\}[^ ].*/        CONTINUATION/g
 		" <"$file" >"$file.fuzzy" &&
@@ -81,7 +82,7 @@ test_expect_success 'pretty format' '
 
 test_expect_success '--abbrev' '
 	sed s/SUBJECT/OBJID/ expect.template >expect &&
-	git shortlog --format="%h" --abbrev=5 HEAD >log &&
+	git shortlog --format="%h" --abbrev=35 HEAD >log &&
 	fuzz log >log.predictable &&
 	test_cmp expect log.predictable
 '

t/test-lib.sh

@@ -166,9 +166,10 @@ esac
 
 # Convenience
 #
-# A regexp to match 5 and 40 hexdigits
+# A regexp to match 5, 35 and 40 hexdigits
 _x05='[0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f]'
-_x40="$_x05$_x05$_x05$_x05$_x05$_x05$_x05$_x05"
+_x35="$_x05$_x05$_x05$_x05$_x05$_x05$_x05"
+_x40="$_x35$_x05"
 
 # Zero SHA-1
 _z40=0000000000000000000000000000000000000000
@@ -184,7 +185,7 @@ LF='
 # when case-folding filenames
 u200c=$(printf '\342\200\214')
 
-export _x05 _x40 _z40 LF u200c EMPTY_TREE EMPTY_BLOB
+export _x05 _x35 _x40 _z40 LF u200c EMPTY_TREE EMPTY_BLOB
 
 # Each test should start with something like this, after copyright notices:
 #