Can git ignore a specific line?

If your file is of a specific type, you can declare a content filter driver, that you can declare in a .gitattributes file (as presented in the “Keyword expansion” of “Git Attributes”):

*.yourType filter=yourFilterName

(you can even set that filter for a specific file, if you want)

Implement:

• yourFilterName.smudge (triggered on git checkout) and

    git config --global filter.yourFilterName.smudge 'sed "s/isPhoneGap = .*/isPhoneGap = true/"'
  

• yourFilterName.clean (triggered on git add)

    git config --global filter.yourFilterName.clean 'sed "s/isPhoneGap = .*/isPhoneGap = false/"'
  

Your file would appear unchanged on git status, yet its checked out version would have the right value for isPhoneGap.

Note: in the comments, ohaal and Roald suggest to isolate the sed calls in a separate helper.sh script:

sed "s/isPhoneGap = .*/isPhoneGap = true/" "$@"