Add support for Magit-Blob buffers

[Miciah]

Use the correct branch or commit when the current buffer is visiting a blob using Magit.

• git-link.el (git-link--commit): Use the magit-buffer-revision variable if the buffer is using Magit-Blob mode. (git-link--branch): Look up the branch using magit-buffer-revision and the magit-rev-branch function if the buffer is using Magit-Blob mode. (git-link--relative-filename): Use the magit-buffer-file-name variable the buffer is using Magit-Blob mode. (git-link--using-magit-blob-mode): New function. (git-link): Set the region and ignore the branch that is checked out into the working tree when using Magit-Blob mode.

[Miciah]

I noticed that it does not include the line numbers when linking from a blob.

Thanks for noticing this, I had missed it. I'll look into it.

[Miciah]

I believe I fixed the missing line numbers. (I'm curious why the when form that I modified is needed at all.)

[sshaw]

Great, thanks again!

[sshaw]

(I'm curious why the when form that I modified is needed at all.)

Because one can link to a file in a dired buffer or in magit buffers and in these cases we don't have an actual file and therefore cannot/should not get the region.