Enforce hard-limit on file size

[trustin]

We currently do not enforce any limit of the size of a file, which can cause troubles such as OOME when a user attempts to push a very large file.

We currently enforce 10 MiB hard limit on request content, but a user may want to increase the maximum allowed content length while limiting the size of an individual file. Think about a commit that contains multiple files whose total size is greater than 10 MiB while their each size is less than 1 MiB.