It sounds like you recently updated GitHub application and Git Shell is now broken.

Short version

To fix it

• close Git Shell
• open GitHub and let it do some post installation.
• Open Git Shell again and you should be fixed.

其实照着上面的做就行了。如果不行的再照着下面的尝试一下哈~~~~（因为我照着上面的做就解决问题了）

Long version

This just happened to me and just to make sure I follow you, you just did this

• Open Git Shell
• You are prompted to update GitHub
• After the update, Git Shell opens

Now it gives the error

git command could not be found. Please create an alias or add it to your PATH.

Warning: Could not find ssh-agent.

If this is the case, do this

• Close Git Shell
• Now open the GitHub application (not Git Shell).

This will say something along the lines of (not sure of the exact version)

GitHub is extracting git..

Let that go through the process and after it is complete, go open Git Shell and it is fixed. Just worked for me at least.