Improve footer parsing to allow multiline footers.

Improve footer parsing to allow multiline footers.

According to the https://git-scm.com/docs/git-interpret-trailers the
CGit supports multiline trailers. Subsequent lines of such multiline
trailers have to start with a whitespace.

We also rewrite the original parsing code to make it easier to work
with. The old code had pointers moving both backwards and forwards at
the same time. In the rewritten code we first find the start of the last
paragraph and then do all the parsing.

Since all the getters of the FooterLine return String, I've considered
rewriting the parsing code to operate on strings. However the original
code seems to be written with the idea, that the data is only lazily
copied in getters and no extra allocations should be performed during
original parsing (ex. during RevWalk). The changed code keeps to this
idea.

Bug: Google b/312440626
Change-Id: Ie1e3b17a4a5ab767b771c95f00c283ea6c300220