[clang-format] formatter only formats changed lines

there is a small issue with the clang-format plugin.

consider this code:

struct foo {
    int   bar;
    float baz;
};

when clang-format's alignment options are used, `bar` and `baz` are aligned.

now, if the code is changed so something like:

struct foo {
 a_really_really_long_type bar;
 float baz;
}; 

saving the file with qt-creator will run clang-format, but not on the full file, but only on the changed lines. so `bar` and `baz` are not aligned, since the line declaring `baz` has not been changed. if this line is modified however, saving the file will align `bar` and `baz` as expected.

i suppose there is some kind of application logic which does not run clang-format on the full file, but only on modified lines. this is a bit surprising to me, as i'd expect it to format the whole file. it smells more like a bug than a feature, but if it's a feature, maybe it can be exposed via an option?