In reference to that last part about having a dedicated subkey for git, I
realized that I should probably just make a separate master key. Please ignore that.

To add, why not also enable forcing a certain subkey without use of the "!"? I
figure that the only reason it's written that way would be in compliance with
the default behavior.

That way, you could make git work better with different subkeys if you wanted to
use a separate subkey dedicated to only signing git commits and tags. Both the
current and the behavior of "newest AND present" wouldn't help if you wanted to
do that, but if you could force a subkey without the "!" then you could easily
have more flexibility in choosing subkeys for git.

I personally am affected by this as well in a couple cases.

This is the topology of my keys:
sec# rsa4096/0x703E78EA22A5ABAB 2015-12-30 [SC] [expires: 2016-12-29]
uid [ultimate] JD Friedrikson (Personal Mail Server)
<me@jdfriedrikson.me>
uid [ultimate] JD Friedrikson (Gmail Address)
<jdfriedrikson@gmail.com>
uid [ultimate] JD Friedrikson (Linode Address)
<jdfriedrikson@linode.com>
uid [ultimate] [jpeg image of size 5874]
ssb rsa4096/0x60E6AFFEEC378639 2015-12-30 [E] [expires: 2016-12-29]
ssb rsa4096/0xC6C7A50DF6FC94C4 2015-12-30 [S] [expires: 2016-12-29]
ssb# rsa4096/0xC5197712F5411047 2015-12-30 [S] [expires: 2016-12-29]
ssb# rsa4096/0x4989B27BD7E45F52 2015-12-30 [S] [expires: 2016-12-29]
ssb# rsa4096/0x04B3529A021FB930 2015-12-30 [S] [expires: 2016-12-29]

I have detached signing subkeys for each device. While I do understand that I
can explicitly force subkey selection with "-u <subkeyid>!" on the commandline
with gpg2, I do not have the option when using programs that are either built as
a front-end for gpg2 (enigmail) or implement gpg in some way (git).

For example, when I try signing a commit with git this is what I get:
λ ~/test/ master* git config --global user.signingkey "0xC6C7A50DF6FC94C4"
λ ~/test/ master* git commit -a -S -m "test"
gpg: signing failed: No secret key
gpg: signing failed: No secret key
error: gpg failed to sign the data
fatal: failed to write commit object

Alright, sure we can try adding the "!" to see if we can force it:
λ ~/test/ master* git config --global user.signingkey '0xC6C7A50DF6FC94C4!'
λ ~/test/ master* git commit -a -S -m "test"
gpg: signing failed: Inappropriate ioctl for device
gpg: signing failed: Inappropriate ioctl for device
error: gpg failed to sign the data
fatal: failed to write commit object

I'm relatively sure that git is having trouble parsing the attempt to force the
subkey.

And if anything else, it does not make sense to me why the default behavior
would be to reach for subkeys that aren't even in the private keyring. I get
that it's going for the newest subkey first, but maybe the behavior should be
newest AND present instead.