Update Checkgit.sh

I did a rough sketch of how this would work. Please consider changing revising where appropriate.

- What does this change do?
Basic checking of sha256 of a file. My change calculates the hash of the file it downloads and also downloads a list of hashes, we then compare the first 64 characters, the hash, that the pc calculates to the hash it downloaded. 

Should there be a mismatch or if a file fails to download, the script will exit 1.  Please change as you feel necessary.

*ps.
I'm not sure if the script should eval the hashes right before it compares them or if the way I evaluate the variables is ok. Also I did not use grep as I have been told it was bad.

SparkleShare/Mac/checkGit.sh

@@ -20,9 +20,19 @@ set -e
 
 if [ ! -f ${projectFolder}/git-${gitVersion}.tar.gz ]
 then
-    curl -s https://codeload.github.com/git/git/zip/v${gitVersion} > git.zip
-    unzip -q git.zip
 
+  list_hash=$(sed -n -e "/git-${gitVersion}.tar.gz/p" sha256sums.asc | cut -c 1-64)
+  file_hash=$(shasum -a 256 git.tar.gz | cut -c 1-64)
+
+  curl -s https://www.kernel.org/pub/software/scm/git/git-${gitVersion}.tar.gz > git.tar.gz
+  curl -s https://www.kernel.org/pub/software/scm/git/sha256sums.asc > sha256sums.asc
+
+  test -e git.tar.gz || {echo "Failed to download git"; exit 1}
+  test -e sha256sums.asc || {echo "Failed to download hash list"; exit 1}
+
+  test "$file_hash" = "$list_hash" || {echo "SHA256 Mistmatch" ;exit 1}
+
+    tar xf git.tar.gz
     cd git-${gitVersion}
 
     make configure
@@ -33,5 +43,5 @@ then
     tar cfz git-${gitVersion}.tar.gz git
     rm -rf git
     rm -rf git-${gitVersion}
-    rm git.zip
+    rm git.tar.gz
 fi