On second thought, git passes filepaths, which may not be valid utf8, so
can't use Text here.
String will be a little bit slower, but not enough to worry about.
Git uses pkt-line in the pack and http protocols, and for the long-running
filter processes protocol as well.
This should be a quite efficient parser and builder since it uses
attoparsec and bytestring-builder.
This adds a dependency on attoparsec, but it's a free dependency because
eg aeson depends on attoparsec and git-annex depends on aeson.
This commit was supported by the NSF-funded DataLad project.
