t: use 'tar' instead of 'zip'

In [1], we used 'zip(1)' to create *.zip files in these integration
tests because ZIP files begin with the header:

    []byte{0x50, 0x4B, 0x03, 0x04}

And thus are detected as 'application/zip' by http.DetectContentType().

However, this 'zip(1)' program does not exist on Windows, hence the
recent test failures (starting as early as [1]).

Instead, let's use 'tar -czf', which creates gzip-compressed tarballs,
and begin with the header:

    []byte{0x1F, 0x8B, 0x80}

Thus are detected as 'application/x-gzip' by http.DetectContentType().

This should allow this test to run on AppVeyor (and more generally, on
Windows as a whole) again.

[1]: 938df5c (tq/basic_upload.go: disable Content-Type detection by
     configuration, 2018-07-31)

t/t-content-type.sh

@@ -10,16 +10,16 @@ begin_test "content-type: is enabled by default"
   setup_remote_repo "$reponame"
   clone_repo "$reponame" "$reponame"
 
-  git lfs track "*.zip"
+  git lfs track "*.tar.gz"
   printf "aaaaaaaaaa" > a.txt
-  zip -j a.zip a.txt
+  tar -czf a.tar.gz a.txt
   rm a.txt
 
-  git add .gitattributes a.zip
+  git add .gitattributes a.tar.gz
   git commit -m "initial commit"
   GIT_CURL_VERBOSE=1 git push origin master 2>&1 | tee push.log
 
-  [ 1 -eq "$(grep -c "Content-Type: application/zip" push.log)" ]
+  [ 1 -eq "$(grep -c "Content-Type: application/x-gzip" push.log)" ]
 )
 end_test
 
@@ -31,17 +31,17 @@ begin_test "content-type: is disabled by configuration"
   setup_remote_repo "$reponame"
   clone_repo "$reponame" "$reponame"
 
-  git lfs track "*.zip"
+  git lfs track "*.tar.gz"
   printf "aaaaaaaaaa" > a.txt
-  zip -j a.zip a.txt
+  tar -czf a.tar.gz a.txt
   rm a.txt
 
-  git add .gitattributes a.zip
+  git add .gitattributes a.tar.gz
   git commit -m "initial commit"
   git config "lfs.$GITSERVER.contenttype" 0
   GIT_CURL_VERBOSE=1 git push origin master 2>&1 | tee push.log
 
-  [ 0 -eq "$(grep -c "Content-Type: application/zip" push.log)" ]
+  [ 0 -eq "$(grep -c "Content-Type: application/x-gzip" push.log)" ]
 )
 end_test