git-gui: Implement "Stage/Unstage Line"

git-gui: Implement "Stage/Unstage Line"

This adds a context menu entry below "Stage/Unstage Hunk" that stages or
unstages just the line under the mouse pointer.

This is by itself useful, for example, if there are unrelated changes in
the same hunk and the hunk cannot be split by reducing the context.

The feature can also be used to split a hunk by staging a number of
additions (or unstaging a number of removals) until there are enough
context lines that the hunk gets split.

The implementation reads the complete hunk that the line lives in, and
constructs a new hunk by picking existing context lines, removing unneeded
change lines and transforming other change lines to context lines. The
resulting hunk is fed through 'git apply' just like in the "Stage/Unstage
Hunk" case.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>