Consonant harmony means that hard consonants and soft consonants are followed by their own type if two consonants come next to each other.

Hard Consonants: p, ç, t, k, h, s, ş, f

Therefore most suffixes that start with a consonant, have two versions. The simple past tense suffix is one of them.

Simple Past Tense Suffixes:

-dı, -di, -du, -dü

-tı, -ti, -tu, -tü

If a verb stem ends with a consonant and that consonant is a hard consonant then we use the "t" version of simple past tense.  

Examples: 

git+ti

koş+tu

Notice that vowel harmony still plays a role. We decide according vowel harmony which one to pick out of these four: -tı, -ti, -tu, -tü.

Thinking about both consonant harmony and vowel harmony put the correct simple past tense suffix to these verb stems:

Example:

git_

your answer: gitti