Have you tried having Mail export the messages to an mbox file?
- Select the messages in Mail.
- File > Save As…
- Format: Raw Message Source
The result should be an mbox file.
In Mail's help, I found the following under “Backing up your email” (first result when searching for “mbox”):
To create an mbox file with just some of your messages, select the messages, then choose File > Save As. In the Format pop-up menu, choose Raw Message Source, then enter a name and save the file.
There are some reports of the resulting file not being in a true mbox format (there are actually several not-quite compatible mbox formats).
The reported problem was that "From " lines in the content of plain-text messages were not escaped. For such lines my version of Mail writes " From " instead of the more typical ">From ", but they are escaped. In the context of emailed Git patches, this should only be a problem for the commit message, but you might look out for it all the same (no matter which path you take to get your emails into mbox format for git am).