Here's what I did. If there's an easier way, feel free to post another answer.
$ git commit
fatal: Unable to create '/cygdrive/c/projectDir/.git/index.lock': File exists.
Another git process seems to be running in this repository, e.g.
an editor opened by 'git commit'. Please make sure all processes
are terminated then try again. If it still fails, a git process
may have crashed in this repository earlier:
remove the file manually to continue.
$ rm /cygdrive/c/Clients/Gulden/dev/Watch_Firmware/.git/index.lock
$ git commit
Found a swap file by the name "/cygdrive/c/projectDir/.git/.COMMIT_EDITMSG.swp"
owned by: ...
file name: /cygdrive/c/projectDir/.git/COMMIT_EDITMSG
user name: ...
process ID: 37252 (still running)
While opening file "/cygdrive/c/projectDir/.git/COMMIT_EDITMSG"
dated: Wed Feb 1 17:11:22 2017
NEWER than swap file!
(1) Another program may be editing the same file. If this is the case,
be careful not to end up with two different instances of the same
file when making changes. Quit, or continue with caution.
(2) An edit session for this file crashed.
If this is the case, use ":recover" or "vim -r /cygdrive/c/projectDir/.git/COMMIT_EDITMSG"
to recover the changes (see ":help recovery").
If you did this already, delete the swap file "/cygdrive/c/projectDir/.git/.COMMIT_EDITMSG.swp"
to avoid this message.
"/cygdrive/c/projectDir/.git/COMMIT_EDITMSG" 16 lines, 547 characters
Press ENTER or type command to continue
vim. Complete and save the message. Then delete the old cached partial message:
$ rm /cygdrive/c/projectDir/.git/.COMMIT_EDITMSG.swp