Git Merge and Fixing Mixed Spaces and Tabs with two Branches

By default git will see each difference in a line's indentation as a change, so yes you're likely to end up with mass conflicts doing a stock merge.

However you can choose the merge strategy to use with the -s option:

git merge -s recursive -Xignore-space-change

This command would use the recursive strategy and uses it's ignore-space-change option. The git-merge docs explain quite well how this will affect your merge:

• If their version only introduces whitespace changes to a line, our version is used;
• If our version introduces whitespace changes but their version includes a substantial change, their version is used;
• Otherwise, the merge proceeds in the usual way

It would also be prudent to look at what git thinks has changed before doing the merge using diff with some extra options. Looking through the diff docs it looks like these options would help you out the most:

-b
--ignore-space-change Ignore changes in amount of whitespace. This ignores whitespace at line end, and considers all other sequences of one or more whitespace characters to be equivalent.

-w
--ignore-all-space Ignore whitespace when comparing lines. This ignores differences even if one line has whitespace where the other line has none.

Why not run both code bases through indent with a the same style on both branches? Doesn't take long.

Then you won't have any whitespace or other issues (like different block styles). Otherwise, yes, it will be hard to merge those branches.

Assuming you code in C, obviously.