Feature Request: Check if commit is existent via http-protocol

Hi there,

for the development of a git client on a SAP System we need to make sure 
that a specific commit is existent in a specific branch. For that we 
have to ask the git server for the related information via the http 
protocol. There are two option from my point of view to achieve this:

1) You can request a specific list of commits of a branch by index (e.g. 
"1 to 30 <sha1 of branch>" would send the first 30 commits from the 
server to the client of the branch "master"
2) You send a request to the git server to verify that a specific commit 
is within a specific branch´and the response is something like "TRUE" or 
the sha1 of the branch the commit belongs to (branch of the time the 
commit was created).

Best regards,

Marius Raht